Pectinoidea (Bivalvia: Propeamussiidae and Cyclochlamydidae) from the Southwestern Indian Ocean

ABSTRACT Twenty-five species of Pectinoidea (24 Propeamussiidae, 1 Cyclochlamydidae) are herein listed from the Mozambique Channel, northwestern and southern Madagascar, and northeastern South Africa. New species: Propeamussium rosadoi, Parvamussium catillus, Parvamussium kilburni, Parvamussium puillandrei, Parvamussium strongae, Cyclopecten cassiculus, Cyclopecten kantori, Cyclochlamys bacachorda. New synonym: Amussium sewelli Knudsen, 1967 = Propeamussium watsoni (E.A. Smith, 1885). New records for the Mozambique Channel and northwestern Madagascar: Propeamussium andamanicum, Propeamussium arabicum, Propeamussium caducum, Propeamussium jeffreysii, Propeamussium sibogai, Propeamussium watsoni, Parvamussium formosum, Parvamussium scitulum, Parvamussium torresi, Parvamussium vesiculatum, Cyclopecten kapalae, Similipecten eous. New records for southern Madagascar: Propeamussium jeffreysii, Propeamussium sibogai, Propeamussium watsoni, Parvamussium formosum, Parvamussium scitulum, Parvamussium thyrideum, Parvamussium vesiculatum, Parvamussium vidalense, Cyclopecten kapalae, Similipecten eous. New record for South Africa: Propeamussium jeffreysii, Parvamussium formosum, Parvamussium scitulum, Cyclopecten horridus, Similipecten eous.

Distribution and habitat: Propeamussium andamanicum is recorded from the Andaman Sea, Laccadive Sea and Arabian Sea (Smith 1895(Smith , 1904, Gulf of Aden and Zanzibar area (Knudsen 1967), New Caledonia (Dijkstra 1995), Vanuatu, Wallis and Futuna (Dijkstra 2001), Fiji (Dijkstra & Maestrati 2008). Now also from northwestern Madagascar and the Mozambique Channel (new record). Propeamussium andamanicum is living on soft substrata of mud or muddy sand at bathyal and upper abyssal depths of -281-2000 m (Dijkstra & Maestrati 2012). Present material live at -1490-1750 m. Remarks: The present specimens resemble in all morphological characters the type material, although the commarginal lirae on the left valve are slightly weaker. Descriptions of the soft parts and diet are given by Knudsen (1967: 274). Description: Shell small, up to 15 mm in height, fragile, transparent, almost circular, somewhat higher than wide, inequivalve, equilateral, left valve slightly more inflated than right valve, auricles slightly unequal, umbonal angle 110°-115°, colour of left valve creamy or reddish, right valve whitish. Prodissoconch 200 µm high. Left valve sculp tured with numerous delicate, regularly spaced, antimarginal lirae, crossed by irregular ly arranged, minutely undulated growth lines, commencing 1 mm below umbo and extending to above central part of disc, central part and periphery smooth. Anterior auricle somewhat larger than posterior with fine commarginal lirae, weaker on posterior. Dorsal margin weakly lamellated near anterior margin. Right valve sculptured with fine, regularly close-set commarginal lirae, more widely spaced near ventral margin. Auricles smooth, unequal in form and size. No byssal fasciole or byssal notch. Ctenolium absent. Hinge line straight. Interior ribs of both valves prominent (7 with 2 auricular) commencing in early growth stage and extending to the submarginal area.
Distribution and habitat: Gulf of Aden, dead at -321-917 m. Now also extended southwards to northwestern Madagascar and the Mozambique Channel (new record). Present specimens are on soft substrata of mud and sand at a bathyal range of -331-825 m (minimum depth range).
Remarks: The most closely resembling species is Propeamussium siratama (Oyama, 1951) recorded from Japan (-100-300 m) (Hayami 2000: 913), which differs from the present species in being smaller (typically up to 10 mm, present specimens up to 15 mm in height), in having a weaker latticed sculpture on the left valve, in having more internal ribs (typically 8 with sometimes a few rudimentary interstitial riblets in late growth stage near the ventral margin, present specimens 7 without rudimentary riblets), and in colour (typically left valve whitish opaque or transparent, present specimens reddish or sporadical whitish transparent). Another resembling species is Propeamussium andamanense (Bavay, 1905) from the Andaman Sea. This species is somewhat smaller in size (up to 9 mm in height), also almost circular of shape and having also 7 internal ribs, but is different in sculpture of the left valve (throughout the disc a fine latticed sculpture of numerous closely spaced radial riblets and weak interstitial commarginal lirae), whereas P. arabicum has a more delicate latticed sculpture in early growth stage to the central part of the disc. Moreover, P. andamanense is more solid, opaque white (P. arabicum transparent reddish or sporadical whitish). A rather resembling species is Propeamussium malpelonium (Dall, 1908), recorded from the tropical eastern Pacific. Propeamussium malpelonium differs in having a more prominent sculpture on the entire disc of the left valve and in having 11 internal ribs. This species is only recorded from abyssal depths, 2690-4505 m (Grau 1959: 14). Propeamussium steindachneri (Sturany, 1901) recorded from the Red Sea, Gulf of Aden, Arabian Sea and Gulf of Oman (-439-457 m) (Knudsen 1967: 280;Oliver 1992: 73;Dijkstra & Janssen 2013: 191) differs from P. arabicum in having a more oblique shape, smooth left and right valves, fewer internal ribs (usually 6), smaller auricles, and creamy and white dots on the left valve.
Propeamussium caducum : Dijkstra 1991: 6, figs 1-2;1995: 15, figs 9-10, 129-132;2013: 10, pl. 1, figs 1a-d, pl. 4, fig. 1; Dijkstra & Maestrati 2008: 82;2012: 391;Dijkstra & Janssen 2013: 187, figs 10-12. Description: Shell up to 29 mm in height, fragile, slightly higher than wide to circular, slightly inequivalve, almost equilateral, semi-transparent to opaque, glossy, creamy, left valve slightly more inflated than right valve, umbonal angle about 90°. Prodissoconch 200 µm high. Left valve smooth, with commarginal growth lines, no radial striations. Auricles small, without sculpture, somewhat raised near margins. Right valve with widely spaced commarginal lirae, commencing at 3 mm shell height and extending to submarginal area, with interstitial microscopic scratches. Auricles with fine commarginal striae, strong scales on the marginal areas of hinge. Hinge line straight near umbo, gradually raised to the anterior and posterior margins. Interior radial lirae generally 10, sometimes 9 or 11, slightly nodulose at distal ends, more prominent on right valve. No byssal notch, ctenolium lacking, lateral gape present. Distribution and habitat: Propeamussium caducum has a large distribution throughout the Indo-West Pacific from southern Japan (Okutani 2000), southwards to the East China Sea and South China Sea (Xu & Zhang 2008), the Philippine Archipelago (Knudsen 1967;Dijkstra 2013), the Indonesian Archipelago (Dijkstra & Kastoro 1997), and eastwards to the Solomon Islands (Dijkstra & Maestrati 2008), the Vanuatu Archipelago and New Caledonia (Dijkstra 1995(Dijkstra , 2001Dijkstra & Maestrati 2012), westwards to the Bay of Bengal, the Arabian Sea and Gulf of Aden, and southwards to Zanzibar area (Dijkstra 1995;Dijkstra & Janssen 2013). Now also extended more southwards to the northwestern area of Madagascar (new record). This species is living on soft substrata of mud or muddy sand at a bathyal range of -90-1500 m (Dijkstra 2013). Present material live at -243-1020 m. Remarks: The present specimens from northwestern Madagascar are almost morphologically completely similar to the type material from the Philippines. Only the present shells are slightly more circular in outline and the commarginal sculpture of the right valve is somewhat more prominent than that of the type specimens. Other characters are identical, i.e. smooth surface of left valve, weak commarginal sculpture on the right valve and in late growth stage on the left valve, number of internal riblets, and coloration and transparency of disc surface. Shell crystallography is given by Hayami (1988), and Knudsen (1967) reported on the soft parts, diet and reproduction.
Propeamussium caducum is the type species of Flavamussium Oyama, 1951, proposed as a subgenus of Parvamussium Verrill, 1897(Dijkstra 1995 (Dijkstra & Maestrati 2008: 83), and westwards to the Maldive Islands, Gulf of Aden and southeastern Africa (Knudsen 1967: 276). Now also recorded from the Mozambique Channel, northwestern and southern Madagascar, and South Africa (new record). Propeamussium jeffreysii is living on soft-bottom assembles of mud or muddy sand at a bathymetric range of -80-797 m (Dijkstra 2013: 13). Present material live at -70-900 m.
Remarks: Almost all morphological characters of the present material are identical to the type specimens from the Philippines. The colour of the type specimens is orangebrownish and of the present material somewhat paler. However, this coloration is also observed in material from the southwestern Pacific (MNHN). The commarginal sculpture on the left valve is somewhat more prominent on the type specimens than of the present material. However, the radial and latticed sculpture of the left valve is variable in prominence or even lacking in a few eroded specimens. The internal ribs are generally 10-11 in number (6-7 main ribs and additional short ribs laterally). The interstitial rudimentary riblets vary strongly (1-4) or are sometimes lacking. Soft parts, reproduction and diet are described by Knudsen (1967: 277).
In the original description of Amussium jeffreysii the right valve is the upper valve and the left valve the lower, in contrast to present descriptions.
Propeamussium rosadoi sp. n. Description: Shell up to 25 mm in height, fragile, most specimens transparent and some opaque, almost circular, inequivalve, slightly equilateral, left valve somewhat more inflated than right. Valves gaping laterally. Auricles relatively small and unequal in size (anterior: posterior ratio is 1.25: 1) and shape (anterior larger than posterior). Umbonal angle about 90°. Colour of left valve orange-brownish tinted, right valve paler, translucent whitish internal radial riblets. Prodissoconch 170 µm long. Left valve in early ontogeny smooth and glossy (c. 3 mm), then sculptured towards the central part of the disc with weak closely spaced radial riblets, in late growth stage smooth with a few delicate commarginal growth lines near the periphery, one or two delicate radial plicae near the posterior margin. Anterior auricle smooth and dull, posterior with microsculpture of very closely spaced radial scratches. Right valve sculptured with regularly spaced (c. 5 per mm) fine commarginal lirae. Anterior auricle laterally with very fine commarginal growth lines, posterior smooth with a narrow groove near the suture of the disc. Interior ribs 9, commencing in early ontogeny and extending submarginally, with one auricular on each side, ribs of right valve broader than of left valve. Resilifer triangular, elongate. Outer ligament rather broad. Hinge line straight. Small byssal notch, no ctenolium. Dimensions: Holotype: Height 25.0 mm, width 24.9 mm, convexity 5.0 mm. Type material: Holotype (pr) (MNHN IM-2000-30115), paratypes (MNHN IM-2000-30117, 16 pr, 5  Distribution and habitat: Northwestern and southern Madagascar. Living bathyally on soft substrata of mud or muddy sand at a maximum depth range of -187-637 m, minimum depth range -247-620 m. Remarks: The most closely resembling species is Propeamussium rubrotinctum (Oyama, 1951) recorded from southern Japan, South China Sea, Solomon Islands, Vanuatu Archipelago, New Caledonia, Fiji, Wallis and Futuna, and Tonga, live at -210-869 m (Dijkstra & Maestrati 2012: 392), which differs morphologically from P. rosadoi in having a smaller size (20 mm in height, P. rosadoi 25 mm), in having a throughout smooth shell disc of the left valve (P. rosadoi partially with a radial sculpture) and a very weak commarginal sculpture on the right valve (P. rosadoi with a more prominent commarginal sculpture), in having 10 and sometimes several rudimentary internal ribs (P. rosadoi 9 without rudimentary riblets), and in having a different colour (orange or whitish patches or dots, P. rosadoi uniform orange-brownish).
Another resembling species is Propeamussium steindachneri (Sturany, 1901) recorded from the Red Sea, Gulf of Aden, Arabian Sea, and Gulf of Aden, live at -363-479 m (Dijkstra & Janssen 2013), which differs from P. rosadoi in having a smaller size (up to c. 15 mm in height, P. rosadoi c. 25 mm), in having a smooth left and right shell disc (P. rosadoi radially (lv) and commarginally (rv) sculptured), and in having fewer and weaker internal ribs (P. steindachneri 6, P. rosadoi 9).
Similar sized specimens of P. jeffreysii from the same stations as P. rosadoi could be easily mixed with P. rosadoi, but the former species could be differentiated from the latter species by the following characters, i.e. closer spaced with rudimentary interstitial internal ribs (P. rosadoi wider spaced and lacking rudimentary interstitial internal ribs), a latticed sculpture on the left valve (P. rosadoi a radial sculpture), a darker orangebrownish coloured left valve, and narrower internal ribs on the right valve (P. rosadoi broader and more prominent).
Remarks: All morphological characters of the present material are identical to the holotype from the Bali Sea, Indonesia. These characters are rather constant and can slightly vary in number of the internal ribs. Occasionally on one or both sides the lateral rib is divided into two separated ribs. The southwestern Indian Ocean morph does not have divided lateral ribs and thus only 6 internal ribs (typically 7).
Propeamussium sibogai is the type species of Luteamussium Oyama, 1951: 82, which was treated by Hertlein (1969: N350) as a junior synonym of Propeamussium, sub sequently followed in literature by most authors (see references). Descriptions of the soft parts, reproduction and diet are given by Knudsen (1967: 272). Description: Shell up to 52 mm high (most specimens smaller up to 40-45 mm), inequivalve, almost circular, slightly higher than wide, somewhat inequilateral, umbonal angle 120°-125°. Opaque, left valve milky-white, right valve cream. Prodissoconch 240 µm high. Left valve more convex than right valve, with delicate radial lirae that commence at c. 3 mm shell length and extend to central part of disc, crossed by commarginal lamellae in central part and extend to the ventral margin, lamellae variable in prominence or even lacking. Auricles relatively small, equal, commarginal lamellae prominent on anterior, finer and more closely spaced on posterior. Most specimens with 10 internal ribs and a small auricular riblet on each side, commencing just below resilifer and extending to pallial line. Right valve with fine regular commarginal lirae and granulate interstitial microsculpture (prismatic calcite layer). Auricles with commarginal lirae, anterior auricle with a few radial lines near suture, absent from posterior auricle. Prominent scales on anterior and posterior dorsal margins of auricles. Resilifer triangular, erect. No byssal notch. Distribution and habitat: Northern Indian Ocean, southern Japan, Papua New Guinea, New Caledonia, Vanuatu, Society Islands, Tuamotu Archipelago, and Austral Islands (Dijkstra & Maestrati 2010: 335;2013a: 362). Now also northwestern and southern Madagascar (new record). Living bathyally on soft substrata of mud or muddy sand at a depth range of -650-2160 m. Present material live at -243-1020 m (maximum bathyal range), minimum range -780-821 m. Remarks: The present material from northwestern and southern Madagascar is almost similar to the type material, although the commarginal lamellae of the left valve are almost lacking (only a few near the ventral margin in some of the largest specimens) and the commarginal lamellae of the right valve are somewhat more widely spaced than the typical specimens. However, both features are variable in examined material from other localities in the southwestern Pacific (MNHN).
There are no basic morphological differences between the Japanese morph P. watsoni bayonnaisense and P. watsoni (Dijkstra 1995: 25).
According to Knudsen (1967: 280) P. sewelli from the Zanzibar region differs from P. watsoni in having a different shape of the auricles and a different reflection of the posterior margin of the posterior auricle. In addition he also observed differences in the "arrangement of the tentacles of the mantle edge" (Knudsen 1967: fig. 18b, c). However, these three characters of P. sewelli are also observed in specimens of P. watsoni from other localities of the southwestern Pacific (MNHN) and are interspecific variations of P. watsoni. Therefore, P. sewelli is treated herein as a junior synonym of P. watsoni. Knudsen (1967: 281) also treated Propeamussium alcocki (E.A. Smith, 1894) as a junior synonym of P. watsoni, whereas Dijkstra (1995: 13) considered both as valid species. Propeamussium alcocki could be distinguished from P. watsoni by having a semi-transparent thin shell (P. watsoni is opaque and more solid), by having a more oval shape (P. watsoni is almost circular), by having a smooth left valve and sometimes some commarginal lamellae in late ontogeny (P. watsoni has a radial sculpture in early growth stage and more prominent commarginal lamellae in later growth stage), and by having thin internal radial ribs (P. watsoni has more solid and broader ribs, especially on the right valve).
Key to species of Parvamussium in the southwestern Indian Ocean -Shell small, circular, semi-transparent, auricles nearly equal, byssal notch moderately deep, 13 internal ribs and one rudimentary .  (Barnard, 1964) Parvamussium catillus sp. n. Fig. 2A-G Etymology: From the Latin catillus (m.) (small dish), after the smooth circular shell that resembles a small dish. Description: Shell up to 7.2 mm in height, fragile, semi-transparent, circular, inequivalve, left valve slightly more inflated than right valve, equilateral, auricles almost equal in size, umbonal angle 110°-115°, colour whitish. Prodissoconch 100 µm long. Left valve smooth and dead throughout shell disc, umbonal part glossy and transparent. Anterior auricle with very weak and delicate widely spaced commarginal lamellae, more prominent near shell disc, posterior auricle almost smooth with traces of very closely spaced commarginal lirae laterally. Right valve with weak, regularly spaced, commarginal lirae (c. 8 per mm). Anterior auricle with weak and very fine commarginal lirae and a noduliferous ridge along the byssal fasciole, posterior auricle continuous with disc, smooth with very close-set commarginal lirae laterally. Byssal notch moderately deep, byssal fasciole rather small, no ctenolium. Hinge line straight. Resilifer triangular. Internal ribs 13 commence in early growth stage and are exposed almost to the periphery, one interstitial rudimentary riblet antero-ventrally, and one riblet on both auricles. Dimensions (holotype): Height 7.2 mm, width 7.2 mm, convexity 2.5 mm.
Another somewhat resembling species is Propeamussium arabicum Dijkstra & Janssen, 2013, recorded from the Gulf of Aden and herein also from northwestern Madagascar and the Mozambique Channel. This species is also circular in shape, but larger sized (up to 14.9 mm in height, P. catillus up to 7.2 mm) and has a fine reticular sculpture on the left valve (P. catillus smooth). Moreover, fewer internal ribs (7 without rudimentary, P. catillus 13 and one rudimentary).
Parvamussium formosum (Melvill in Melvill & Standen, 1907) Melvill & Standen, 1907: 807, pl Remarks: The present specimens from northwestern and southern Madagascar and South Africa are almost identical to the type material. The present specimens are slightly different in having a more coarse commarginal sculpture on the anterior auricle of the left valve (typically more delicate and closer spaced), and in having in a few specimens faint commarginal sculpture on the central disc shell of the left valve (in typical specimens lacking), but other morphological characters are similar.
Smooth juvenile specimens of P. formosum and P. siebenrocki could be easily confused, although the latter species has more prominent radial and wider spaced commarginal sculpture laterally and on the auricles.
A closely resembling species is Parvamussium thetidis (Hedley, 1902), recorded from tropical eastern Australia, the Coral Sea as far east as the New Hebrides Archipelago. This species is living bathyally deeper at -500-650 m (Dijkstra 1995: 35). Parvamussium thetidis has a delicate radial and commarginal sculpture on the left valve (P. formosum almost lacks commarginal sculpture), although the radial riblets are more prominently developed and the commarginal lamellae more closely spaced throughout the shell disc, and fewer internal ribs (P. formosum 10 + 1-7 rudimentary, P. thetidis 10 + sometimes one rudimentary).
Parvamussium kilburni sp. n.   (Melvill in Melvill & Standen, 1907), recorded from the northwestern Indian Ocean. This species has prominent radial and commarginal sculpture commencing in early growth stage on the left valve (P. kilburni lacks radial sculpture and has a commarginal sculpture only developed marginally in late ontogeny). Parvamussium formosum has also more prominent interior ribs with more interstitial rudimentary riblets (up to 7) in adult specimens.
Parvamussium puillandrei sp. n. Description: Shell up to 12.9 mm in height, fragile, inequivalve, inequilateral, somewhat higher than wide, left valve slightly more inflated than right valve, whitish, auricles almost equal in size, relatively small, umbonal angle c. 90°. Prodissoconch 210 µm long. Left valve with prominent commarginal lamellae, commencing c. 0.5 mm below the umbonal top and covering the shell disc (c. 6-7 per mm on central part of disc) towards the ventral margin, crossing c. 10-15 irregularly spaced radial costae in early growth stage, commencing c. 2 mm below the umbonal top, increasing by interstitial secondary radial riblets towards the ventral margin to c. 30 in number. Auricles with closely spaced commarginal lamellae, which is somewhat more prominent on the anterior auricle. Hinge line straight. Right valve with regularly spaced slightly undulated commarginal lirae, weak in early ontogeny, more prominent laterally, covered with radially arranged fine vesicular sculpture. Anterior auricle well developed, sculptured with 6-7 fine radial riblets, covered with commarginal lamellae, which are strongly developed on antero-dorsal margin.
Posterior auricle covered with commarginal lamellae, which are more prominent on postero-dorsal margin. Marginal apron broken off in all specimens seen. Internal lirae 10, 3 of them rudimentary, plus 1 small posterior auricular lira on the left valve; 9 lirae plus 3 rudimentary, and 1 posterior auricular lira on right valve. Lirae commencing in early growth stage and enlarging towards the submarginal area (c. 3 mm from the periphery). Exterior sculpture of left valve slightly visible from the interior. Outer ligament small. Resilifer triangular. Byssal notch small. Fasciolar pseudo-ctenolium lacking. Dimensions: Holotype: Height 12.9 mm, width 12.3 mm, convexity 3.0 mm. puillandrei up to c. 20, P. undisonum up to c. 15) and more interstitial secondary radial riblets, in having a broader groove between the disc and the anterior auricle of the right valve (P. undisonum narrow), in lacking a pseudo-ctenolium (which is present in adult P. undisonum), and in having earlier developed internal ribs (internal ribs in P. undisonum commence in a later growth stage near the adductor insertion scar).
The pseudo-ctenolium, an unusual character in Propeamussiidae, is discussed by Waller (1984: 213, figs 5a-b) in Parvamussium sayanum (Dall, 1886), known from the tropical western Atlantic, and this feature is also observed in adult specimens of P. undisonum, lacking in the present species.  Distribution and habitat: Southern Japan southwards to Australia, eastwards to the Austral Islands (Dijkstra 2013: 22), and now also westwards into the Indian Ocean from the Mozambique Channel, northwestern and southern Madagascar and northeastern South Africa (new record). Living free sublittorally and upper bathyally (Dijkstra & Maestrati 2012: 393) on soft substrata (mixtured sediments of sand, muddy sand or mud with coral rubble or gravel). Present specimens live at -53-112 m (minimum depth range). Remarks: Kuroda (1929Kuroda ( -1935) described a possible new variation of the present species from Japan, which is more brightly coloured and much larger in size than the type material of Parvamussium scitulum. Smith (1885: 312) already suggested that the type specimens from Papua New Guinea are possible juveniles. The present specimens from northwestern and southern Madagascar and the Mozambique Channel are indeed larger in size (up to 9 mm in height, typically 4.5 mm), more colourful (typically whitish), and some specimens have one more internal rib, but other morphological characters are identical to the type material.
Parvamussium siebenrocki (Sturany, 1901) Fig . 4D    Remarks: A closely smooth bathyal species is Parvamussium torresi (E.A. Smith, 1885), recorded from the southwestern Pacific. This species could be distinguished from P. strongae by the following characters, i.e. in having a smaller size (P. torresi up to c. 10 mm in height, P. strongae c. 27 mm), in coloration (P. torresi is uniform whitish, P. strongae creamy with whitish and brownish maculations), and in development of internal ribs (P. torresi 10 well developed and prominent, P. strongae 14-16 rudimentary). Two congeneric species with also similar anteriorly and posteriorly rudimentary internal riblets as in the present species are Parvamussium vidalense (Barnard, 1964), recorded from South Africa, and Parvamussium vesiculatum Dijkstra, 1995, known from the southwestern Pacific. These species could be distinguished from P. strongae in having a significant smaller size (up to c. 8 mm in height), in having a well-developed radial and commarginal sculpture on the left valve, and in having fewer rudimentary internal riblets (generally 3-4 anteriorly and posteriorly).
Parvamussium texturatum (Dautzenberg & Bavay, 1912) Fig. 5D-F Amussium texturatum Dautzenberg & Bavay, 1912: 37, pl. 27, figs 19-22;Barnard, 1964: 432.  Description: Shell up to 8 mm in height, fragile, almost circular, nearly opaque, weakly inflated, inequivalve, inequilateral, valves almost equally convex, auricles unequal in shape and size (anterior much larger than posterior), umbonal angle 95°-110°. Colour whitish or creamy with a few white or brown spots. Prodissoconch 200 µm high. Left valve only sculptured with prominent commarginal lamellae in early growth stage (pre-radial stage below dissoconch). Irregularly arranged radial costae (c. 10) commence after 1-2 mm below the umbonal top, increasing by secondary intercostal radial costulae, enlarging towards the ventral margin. Strongly developed commarginal lamellae are crossing the radial costae, forming more or less prominent squamae on the intersections. Auricles with very prominent and close-set commarginal lamellae, somewhat weaker on posterior auricle. Right valve sculptured with regularly spaced, somewhat scaly, prominent commarginal lamellae. Anterior auricle with c. 4-7 radial riblets and weak overrunning commarginal lamellae. Posterior auricle with prominent close-set commarginal lamellae. Internal ribs, variable in length (in most specimens short) and number (c. 10 + 1-6 rudimentary), commencing generally in late ontogeny. Byssal notch narrow. Remarks: The present specimens are morphologically close to the holotype of the Sulu Sea, although the intercostal commarginal lamellae on the left valve are weaker, the scaly commarginal lamellae on the right valve lacking, and the internal ribs commencing in an earlier growth stage. However, the sculpture and also the internal ribs are rather variable and these variable characters are also examined in material from the southwestern Pacific (MNHN). A morphologically close congener is Parvamussium conspectum Dijkstra & Kastoro, 1997, known from eastern Indonesia. This species differs from P. texturatum by having a more elongated shape (P. texturatum is almost circular), coarser commarginal sculpture in early growth stage, and fewer internal ribs (P. conspectum 9, P. texturatum 10 + 1-6 rudimentary).
Parvamussium thyrideum (Melvill in Melvill & Standen, 1907)  Description: Shell up to 5 mm in height, flattened, subcircular, left valve slightly more convex than right valve, opaque or semi-transparent, inequivalve, equilateral, auricles unequal, umbonal angle 110°, colour whitish or creamy with white dots. Prodissoconch 140 µm long. Left valve sculptured with 13-17 prominent, irregularly spaced, radial costae, commencing c. 1 mm below umbonal top and extending to the ventral margin, and weaker, more regularly and widely spaced commarginal lamellae, with fine scales on intersections. Secondary interstitial radial riblets are developed marginally. Distribution and habitat: Red Sea, Gulf of Aden, Arabian Sea and Persian Gulf. Now also from southern Madagascar (new record). Upper bathyal depths. Living on soft substrata (mud, muddy sand or sand with shell grit). Present specimens dead at -80-220 m. Remarks: The present specimens of southern Madagascar are almost similar to the type material. The specimen of stn DW3519 has internal rudimentary radial riblets, which are lacking in the type material. These riblets are strongly variable and only developed in adult specimens. A close congener is Parvamussium cancellus (Dijkstra, 1991), recorded from the southwest Pacific, which differs morphologically from P. thyrideum by having a more prominent reticulate sculpture on the left valve with fewer intercalated secondary radial riblets.

Distribution and habitat: Southern
The right valve of P. torresi is sculptured with weak commarginal lirae, which are lacking from most specimens of P. scitulum. The present material from the Mozambique Channel is morphologically almost identical to the type specimens from Torres Strait (NE Queensland), but are slightly different in colour (with reddish or creamy dots, typical specimens are whitish), and have a prominent transparent and glossy shell disc in early growth stage of the left valve (typical specimens are more dull).
Distribution and habitat: Southwestern Pacific from Taiwan southwards to Australia, eastwards to Tonga (Dijkstra & Maestrati 2008: 96), now also extended into the southwestern Indian Ocean from northwestern and southern Madagascar (new record). Living bathyally on soft substrata of mud and sand at -205-650 m. Present specimens live at -200-220 m. Remarks: The present specimens from northwestern and southern Madagascar are morphologically identical to the type material, although one left valve is almost smooth and transparent and another left valve almost lacks the typical hollow scales on the intersections and has prominent internal ribs, which commence in early ontogeny. However, these characters are strongly variable and similar specimens are examined in material from the southwestern Pacific (MNHN).
Parvamussium vidalense (Barnard, 1964)  Remarks: The present specimen from southern Madagascar is morphologically identical to the type specimen. Parvamussium vidalense was provisionally placed in Cyclopecten by Barnard (1964: 433), but developed internal riblets justified placing in Parvamussium.
A close congener from the SW Pacific is Parvamussium araneum Dijkstra, 1991, which differs from P. vidalense by having finer and more closely spaced commarginal lamellae, more delicate radial riblets on the left valve, more spineous sculpture on the intersections, and less-developed internal riblets.  (Barnard, 1964) Remarks: For synonymy, diagnosis, distribution and discussion see Dijkstra (1995: 41) and Waller (2011: 17).
Key to species of Cyclopecten in the southwestern Indian Ocean -Shell small, somewhat higher than wide, lv with prominent radial and weaker commarginal sculpture, intersections with nodules or lamellae, internal ribs lacking .  Dijkstra, 1990 Cyclopecten cassiculus sp. n. Fig. 6A-G Etymology: From the Latin cassiculus (m.) (small spider's web), after the sculpture of left valve that resembles a spider's web with fine raindrops on the intersections. Description: Shell up to 8.9 mm in height, fragile, lv opaque, rv semi-transparent, somewhat higher than wide, inequivalve, flattened and somewhat rough, left valve slightly more convex than right valve, inequilateral, anterior auricle larger than posterior, umbonal angle 90°-95°, colour of left valve creamy, right valve whitish. Prodissoch 170 µm long. Left valve sculptured with very closely spaced commarginal lirae in early growth stage (c. 10 per mm), more prominent and widely spaced on central shell disc (c. 4-5 per mm), 8-10 fine radial lirae commence c. 0.8-1.0 mm below umbo, increasing to c. 15 strong primary lirae near periphery with interstitial secondary lirae, intersections with small nodules on primary radial lirae in early growth stage, more lamellar in adult stage, small nodules on secondary interstitial riblets, similar sculptured auricles almost continuous with shell disc. Right valve with very weak, regularly spaced, delicate commarginal lamellae (c. 10 per mm). Anterior auricle with 6-7 antimarginal noduliferous lirae, posterior auricle continuous with disc with very fine and close-set commarginal lirae. Byssal notch moderately deep, byssal fasciole rather small, no ctenolium. Hinge line straight. Resilifer triangular. Internal ribs lacking. Dimensions (holotype): height 8.9 mm, width 8.0 mm, convexity 2.9 mm.
Type material: Holotype, pr (MNHN IM-2007-41951), paratype, pr (MNHN IM-2007-41954 Dijkstra, 1991, re corded from deep water (-450-600 m) off Indonesia. Cyclopecten cassiculus morphologically differs from P. cassium by having prominent radial and fine commarginal lirae (in P. cassium weaker and more equal of prominence), in having secondary intercostal radial lirae in late ontogeny (P. cassium in earlier growth stage), in having fine interstitial radial lirae, up to 6 near ventral margin (one radial lira in P. cassium), and in having coarser nodules and lamellae on intersections of lirae (P. cassium very small nodules). Both species are lacking internal ribs, although P. cassium has a short auricular riblet (very weak in the present species).
A somewhat resembling species is Parvamussium vidalense (Barnard, 1964), recorded from eastern South Africa and herein also from southern Madagascar. This species has a reticular sculpture on the left valve of more closely spaced, finer radial and commarginal lirae with short spines or curved lamellae on the radial and commarginal intersections. Moreover, several rudimentary interior ribs are present (lacking in C. cassiculus).  Description: Shell up to 11.9 mm in height, fragile, subcircular, inequivalve, almost equilateral, left valve more inflated than right valve (nearly flat), auricles dissimilar in shape, anterior auricle larger than posterior one, umbonal angle 120°, internal ribs lacking, semi-translucent whitish. Prodissoconch 220 µm long. Left valve reticulately sculptured throughout shell disc by narrow radial riblets and overrunning commarginal lirae with delicate solide nodules on intersections. On central part of disc c. 10 radial riblets per mm, c. 7-8 commarginal lirae. Reticular sculpture commences in early growth stage, radial riblets enlarge gradually in ontogeny by intercostal riblets towards the ventral margin. Commarginal lirae more prominent than radial riblets, very closely spaced in early growth stage, gradually wider in later ontogeny. Auricular sculpture similar to that of shell disc with stronger solid nodules on the intersections. Right valve sculptured with commarginal lirae, closely spaced (c. 30 per mm) near umbonal area, coarser and wider on central part of disc (c. 8 per mm). Posterior auricle continuous with shell disc, sculptured with fine close-set commarginal lirae, anterior auricle and byssal fasciole seperated from shell disc by sharply incised groove, sculptured with prominent commarginal lirae and 8 crowded radial riblets with nodular intersections. Byssal notch moderately deep, byssal fasciole narrow. Dimensions: Holotype: Height 11.9 mm, width 11.1 mm, convexity 3.2 mm.
Type material: Holotype, pr (MNHN IM-2007-38440 Dijkstra & Marshall, 2008, recorded from the Lord Howe Rise. This species could be distinguished from C. kantori by the following characters: by shell shape (C. textus is almost circular, C. kantori somewhat higher than wide), by shape of auricles (C. textus has smaller auricles (c. 2 mm) measured between end of hinge and disc, C. kantori somewhat broader (2.5 mm)), by commarginal sculpture of the left valve (C. textus has less commarginal sculpture in early ontogeny, P. kantori more close-set), and by reticulate sculpture in the central part of the disc of left valve (C. textus more squarely, C. kantori more elongate). A somewhat similar reticulated sculptured bathyally living species is Parvamussium retiolum Dijkstra, 1995, recorded from the southwestern Pacific. This species could be easily distinguished from C. kantori by its more oblique elongate shape (C. kantori sub-circular), and by its well-developed internal ribs (lacking in C. kantori).
Left valve with reticulate sculpture of weak unevenly spaced radial costae and slightly more prominent commarginal lamellae, interstices microscopically scratched. Radial and commarginal sculpture commence in early ontogeny, c. 1 mm below umbonal top. Radial riblets increasing in prominence towards ventral margin. Anterior auricle with coarser commarginal lamellae than posterior one, lacking radial riblets. Right valve with commarginal lamellae, somewhat higher and coarser at ventral margin than higher up, interstices with microscopic scratches. Anterior and posterior auricles also with commarginal lamellae, slightly more irregular on anterior one. Byssal fasciole small, byssal notch hardly distinguished from outer margin of anterior auricle. Hinge line almost straight. Hinge dentition with strong irregular grooves. Internal ribs lacking. Distribution and habitat: Solomon Islands (Dijkstra & Maestrati 2008: 97), southeastern Australia (Dijkstra 1990b: 29), Norfolk Island and Kermadec Islands (Dijkstra & Marshall 1997: 17), Vanuatu (Dijkstra & Maestrati 2012: 393). Now also extended into the southwestern Indian Ocean, from the Mozambique Channel and southern Madagascar (new record). Living bathyally (-512-914 m) on soft substrata of mud and sand with sediments. Present specimens live at -707-708 m (minimum depth range). Remarks: Present specimens from the Mozambique Channel and southern Madagascar are morphologically indistinguishable from the type material. The reticulate sculpture on the left valve is somewhat variable in prominence of the present material, but this feature is also observed in material from the southwestern Pacific (MNHN).