The Kishenehn Formation of northwest Montana and southeast British Columbia hosts one of the most prolific and diverse assemblages of fossil nonmarine mollusks in North America. More than 76 taxa, of which 41 are newly described species, of middle Eocene to early Miocene terrestrial and aquatic gastropods have been collected and described. This paper reports on the discoveries made in the oldest part of the formation that is middle Eocene in age in the southernmost part of the Kishenehn Basin. Outcrops along the Middle Fork of the Flathead River from this area have yielded over 1400 specimens from which 62 taxa (37 terrestrial, 25 aquatic) were identified. Of these, eight are new species and nine were described elsewhere but were not previously reported in the formation.
In terms of paleoenvironmental affinities, the fauna are divisible into three groups. Group I is a relict tropical wet fauna, existing in basinal waterways whose modern analogs live in the Caribbean and Central and South America. Group II is a subtropical, semi-arid fauna with modern analogs ranging from the north coast of the Gulf of Mexico, across the southern United States and northern Mexico to southern California and the Baja Peninsula. Group III is a humid, warm temperate fauna that was transported from mountains east of the basin and deposited in fan-delta and lake-margin settings. Modern analogs of these molluscan taxa are common in the east-central United States. The unusual coexistence of these climatically disparate groups of fossil mollusks is a manifestation of the large paleotopographic relief, perhaps greater than 2000 m, that existed between the valley floor and the mountains.
The following taxa are described herein by the senior author (HGP) as new species: Bivalvia: Sphaeriidae- Sphaerium lentum, Pisidium triangulatum. Gastropoda: Proserpinellidae- Tozerpina elsae; Subulinidae- Obeliscus scalplockensis; Planorbidae- Ferrissia circus, Rhodacmaea crepida, Omalodiscus spira; Physidae- Physella olive; and one new subspecies: Gastropoda: Pupillidae- Pupilla hebes sinistra.
