Circular 194—Pecosorbis, a new genus of fresh-water snails (Planorbidae) from New Mexico

By D. W. Taylor, 1985, 17 pp., 1 table, 14 figs.

This species was known previously only as a Pliocene fossil, but now is recognized in the Quaternary of the southwest U.S. and is living in the Pecos Valley of New Mexico. Pecosorbis is unusual because of its restricted distribution and habitat in seasonal rock pools. Pecosorbis, new genus of Planorbidae, subfamily Planorbulinae, is established for Biomphalaria kansasensis (Berry). Most similar to Menetus, it differs in having a perputial organ with an external duct, no spermatheca, and a penial sac that is mostly reversible.

The novel discovery described in this circular is significant in various ways. Within the context of molluscan studies, scarcely anyone would have expected this new genus in the U.S. Fresh-water snails of the family Planorbidae have been studied for a long time in North America, and Pecosorbis is markedly distinct in taxonomic characters. Its habitat in seasonal pools and its restricted distribution are unusual. Furthermore, it proves to be a "living fossil," a Pliocene species thought to be extinct. Although several Pleistocene species of fresh-water molluscs have been discovered alive, this is the first occasion in North America when a Tertiary form has been recognized as living. Once again shell features in Planorbidae are shown to give little clue to relationships, and the original generic allocation of the fossils is revised significantly.