Remarks.—Angusticornids are characterized by the acutely angular lateral transition between the flattened ventral surface and the convex dorsal surface (Malinky and Geyer, 2019).

Type species.—Linevitus distortus Syssoiev, 1962 from the Cambrian (late Terreneuvian) of the Aldan River, Siberia, by original designation.

Remarks.—Missarzhevsky (1969, 1981) proposed Burithes from the Tommotian and Atdabanian stages (Cambrian Stage 2 and 3) of Siberia, with type species Linevitus distortus Syssoiev, 1962 from the Aldan River. The shallowly convex dorsum, considered ventral by Missarzhevsky (1969, 1981), passes by way of acute lateral angulations onto the almost flat ventral surface. Malinky (2014) noted that all specimens in Syssoiev's (1962) type material were internal molds and that details of ornamentation and the external expression of the lateral edges were not known. Consequently, Malinky (2014) recommended that the name should be restricted to the type material, and he further established Haydenoconus, in which the lateral edges on the shell exterior are sharp, keel-like. However, the type species, Hyolithes gallatinensis Resser, 1938b, was described from the Furongian of Wyoming although a second species, Haydenoconus prolixus (Resser, 1939), was described from the Miaolingian of Idaho. Malinky (2014) did not comment on the status of the other species that Missarzhevsky (1969, 1981), Meshkova (1974), Meshkova et al. (1983), and Rozanov et al. (2010) referred to Burithes. While acknowledging Malinky's (2014) recommendation, Burithes is employed in the present context on the basis of the descriptions and illustrations presented by Missarzhevsky (1969, 1981).

Description.—Conch with incremental angle 20°–25° and ligula about one-sixth of total length (Fig. 2.1), with slight longitudinal curvature such that the ventral surface may have been shallowly convex (Fig. 2.3). Width of ligula about half its length, but length increasing proportionately with growth (Fig. 2.4). Shallow lateral sinuses for the likely passage of helens lie on the dorsal side of the angular transition from the convex dorsal surface to the almost flat ventral surface. Dorsal surface seemingly uniformly shallowly convex, but degree of inflation uncertain due to crushing. Ornamentation on ventral surface of fine comarginal growth lines with occasional growth halts that may appear periodic (Fig. 2.3). Ornamentation on dorsal surface poorly known, seemingly almost orthocline. Operculum and helens not certainly known, but associated isolated opercula are wider than long (Fig. 3), supporting the interpretation that the dorsal surface of the conch was not strongly inflated.

Material.—PMU 35719–PMU 35724, Murray Shale, Montezuman Stage (Cambrian Stage 3), Chilhowee Mountain, Tennessee.

Remarks.—All specimens are compressed, although a degree of separation between the dorsal and ventral surfaces may be maintained (Fig. 2.4). Crushing has often produced Y–shaped cracks that extend from the ligual margin down the median line as a raised, irregular ridge or angulation to near the apex (Fig. 2.1). The median fracture is commonly expressed as a ridge on the ventral surface, but this is a preservational artifact. However, a broad, rounded ridge in some specimens (Fig. 2.1) likely represents compaction around a solid object within the conch interior, possibly an early mineralized burrow or sediment-infilled section of gut (Devaere et al., 2014).

In terms of its overall shape, the Murray Shale conchs are similar to Burithes erum Missarzhevsky, 1969 from the Tommotian of the Anabar Massif of Siberia (Missarzhevsky, 1969, 1981). Nevadotheca whitei (Resser, 1938b) from the Pioche Shale (Cambrian Series 2) of Nevada, the type species of Nevadotheca Malinky, 1988, differs in having a high, inflated dorsum and narrowly rounded lateral margins. This is also the case in Nevadotheca boerglumensis Peel and Willman, 2018 and Kalaallitia myliuserichseni Peel and Willman, 2018, described from the early Olenellus Biozone (Dyeran, Cambrian Stage 4) of the Buen Formation of southern Peary Land, North Greenland, but Kalaallitia is distinguished by its fine longitudinal lirae and longer ligula. Opercula referred to these Peary Land species are proportionately longer than Murray Shale specimens, suggesting that the dorsal surface of their conchs was more strongly inflated than in Burithes? sp.

Malinky and Skovsted (2004) noted some similarity between specimens from the Dyeran of North-East Greenland and Burithes, but the acute dorsum of their material suggested assignment to Grantitheca Malinky, 1989 by Peel and Willman (2018). In contrast to material from the Murray Shale and Buen Formation, hyoliths from North-East Greenland are preserved mainly as internal molds in limestone or as phosphatic residues from limestones (Malinky and Skovsted, 2004; Skovsted, 2006).

Material.—PMU 35725, internal mold and corresponding external mold of the ventral surface, Murray Shale, Montezuman Stage (Cambrian Stage 3), Chilhowee Mountain, Tennessee.

Remarks.—This species, known from the illustrated specimens and two additional fragments, is characterized by two or three prominent transverse folds or corrugations on the adapertural part of the shallowly convex ventral surface of the conch and ligula (Fig. 2.5, 2.9). The incremental angle is about 30°, and the length of the ligula is slightly more than half its width. Lateral sinuses are present at the transition from the shallowly convex ventral surface to the dorsal surface, but the degree of inflation of the latter is not known. Ornamentation consists of fine comarginal growth lines, although these are more strongly developed at the preserved aperture (Fig. 2.5).

Corrugation of the latest growth stage of the ventral surface and ligula is common in hyolithids, but not with the high degree of emphasis seen in the Murray Shale specimen (Fig. 2.9). Examples were illustrated by Missarzhevsky (1969, 1981) in specimens from Siberia referred to Trapezovitus sinscus (Syssoiev, 1958) and Burithes cuneatus Missarzhevsky, 1969, by Qian et al. (2003) in Nitoricornus wushiensis Qian et al., 2003 from the lower Cambrian of Xinjiang, China, and by Malinky (1990, fig. 1.6) in specimens from New Brunswick.

Simpson and Sundberg (1987) interpreted a specimen from the Hampton Formation of southwestern Virginia as a rugose hyolith compared to Tuojdachithes? biconvexus (Cobbold, 1919), but Hageman and Miller (2016) considered that it was not a fossil. Peel and Willman (2018, fig. 13G) illustrated a more-slender, unnamed hyolithid from Buen Assemblage 1 at Brillesø, southern Peary Land (Fig. 1.1) with rugose comarginal ornamentation on the ventral surface of the ligula, but this is much less prominent than in the Murray Shale specimens.

Material.—PMU 35727–PMU 35728, Murray Shale, Montezuman Stage (Cambrian Stage 3), Chilhowee Mountain, Tennessee.

Remarks.—The coincidence of the long axes of three overlapping incomplete specimens in ventral aspect (Fig. 2.12) suggests that they may form part of a gut fill or coprolite (Vannier, 2012). One of the specimens preserves the operculum in place (Fig. 2.11, arrow in 2.12). The incremental angle is about 20°, and the ligula is short. The ventral surface is shallowly convex, but the lateral edges are rounded and delimited on their axial edge by a shallow longitudinal furrow. The dorsal surface is not known, but lateral sinuses seem to be well developed (Fig. 2.11). The impression of the dorsal exterior of the operculum shows comarginal growth lines and a suggestion of a radial furrow. While similar in shape to that illustrated in Figure 3.5–3.7, it is too poorly preserved for closer comparison.

Material.—PMU 35726, Murray Shale, Montezuman Stage (Cambrian Stage 3), Chilhowee Mountain, Tennessee.

Remarks.—This poorly preserved specimen has an incremental angle of 15° and appears to have an oxygonal aperture. Adaperturally, shallowly convex comarginal growth lines on the ventral surface are cord-like and laterally discontinuous, resembling the pattern illustrated by Malinky (1990) as Nitoricornus danianum (Matthew in Walcott, 1884) and by Kruse and Hughes (2016) as ?Crestjahitus danianus from New Brunswick, Canada, whereas the dorsal surface is ornamented with barely discernible fine growth lines.

Material.—PMU 35729–PMU 35731, Murray Shale, Montezuman Stage (Cambrian Stage 3), Chilhowee Mountain, Tennessee.

Remarks.—Three poorly preserved specimens of partially articulated hyolithid skeletons occur (Fig. 2.11, 2.12), but several isolated opercula are known (Fig. 3). The length of the best-preserved specimen is about four-fifths of its width (Fig. 3.5). Following terminology employed by Malinky and Berg-Madsen (1999, text-fig. 2), its conical shield is hemispherical in plan view, and the summit lies at about one-quarter of the distance from the adapical margin to the adapertural margin (Fig. 3.5). The folds separating the conical and cardinal surfaces delimit an angle of about 130° in plan view (Fig. 3.5). In lateral perspective (Fig. 3.7), the cardinal shield rises high above the summit of the operculum, with the inclination of its adapical margin (right in Fig. 3.7) suggesting that the conch had an amblyogonal margin. The cardinal area is ornamented with radial ridges transverse to the comarginal growth lines present over the entire conch. The latter are most conspicuous on the conical shield where radial ornamentation consists of fine lines (Fig. 3.5, 3.6).

A second specimen, preserved as an external mold, has a well-defined convex adapical border with prominent coarse ridges between the summit and the adapical margin (Fig. 3.4). A broad shallow sinus in the margin (Fig. 3.4, arrow) marks the exit point for the helen. A third specimen (Fig. 3.1–3.3) has a more elliptical shape than the other two specimens and develops a series of short comarginal rugae located medially on the conical shield.

The cardinal surface is proportionately longer in opercula of Nevadotheca boerglumensis and Kalaallitia myliuserichseni from the early Olenellus Biozone (Buen Assemblage 2) at Brillesø, southern Peary Land (Peel and Willman, 2018, fig. 14), and the folds separating the conical and cardinal surfaces delimit an angle of about 90°, much smaller than in the specimens from the Murray Shale. In this respect, the opercula from the Murray Shale more closely resemble the opercula associated with articulated specimens from the Sirius Passet Lagerstätte (Fig. 4.2, 4.4), but these are too poorly preserved for close comparison. A similar wide angle is seen in Nitoricornus danianum illustrated by Malinky (1990, fig. 1.1) from New Brunswick and assigned to ?Crestjahitus danianus by Kruse and Hughes (2016).