The Vel iger 4 6(4):275- 304 (October 6. 200 3) THE VELIGER © CMS , lnc., 2003 Rissooidean Snails from the Pit River Basin, California ROBERT HERSHLER D epartment of Systematic Biology, Smithsonian Institution, P.O. Box 37012, NHB W- 305 , MRC 163, Washington, D.C. 20013 -70 12 , USA TERRENCE J. FRES T DEIXI S Con sultants, 2517 NE 65th Street, Seattle, Washington 98115-7125, USA HSIU-PING LIU D epartment of Biological Sciences, Univer sity of Denv e r, De nver , Colorado 80208, USA AND EDWARD J. JOHANNES DEIXI S Consultant s, 2517 NE 65th Street, Seattle, Washington 98115- 7125 , USA Abstract. A Tecent field s urvey of the P it River basin in northeastern California re sulted in the di s covery of numerou s undescTibed populations of aquatic ri ss ooidean snails which are tre at ed in this pap e r. Ba se d on morphologi ca l study and analysis of mitochondrial DNA sequences we describe a new sp ecies of the genus Colli gy rus (family Amnicolida e) and four new s pecies of the genus Py rgulopsis (HydTobiidae). Phylog enetic analysis of s equenc e data suggests that the new species of Collig y rus is sister to an undescribed congener from the Kl amath basin. Th e new spe cie s of Py rg ulopsis do not form a· monophyletic gToup but in ste ad are variously re lat ed to oth er re gional congeners. We also d es cribe P it Ri ver ba s in population s· of two s pecies of Py rgulopsis which were previously cons idered to be end e m.ic to the north- western Great Basin (P. ere mica) and Klamath bas in (P. archimedis ). INTRODUCTION Rissooidean snails are one of the richest element s of aquatic biota in the western United States, with more than 170 species currently recognized, yet this fauna remains poorly known. Most of the previously de s cr ib ed member s of tills fauna have been little studied and are in need of taxonomic rev ision. Exi sting collections document a large number of unde scribed species, and discovery of addi - tional novelties is anticipated as large expanses of the West still have not been thoroug hly s urveyed for these small, nondescript , highly speciose anjmals. One little ex- plored region is the P it River ba s in of northeastern Cal - if ornia and southeastern Oregon, which is the major head- waters tributary of the Sacramento River. The only ris- sooidean snails that have been previously reported from this ba s in .in the taxonomic literature are two speci es of Fluminicola (Hydrobi id ae). One of thes e, F. s emina/is (Hinds, 1842), is now extirpated fr om much of it s hi storic range but per sists in a few large sp1ings and streams (Tay- lor , 1981; Her shler & Fres t, 1996) while the other , F. modoci Hannib al, 191 2, live s in springs along the margin of Goose Lake (Hershler & Fr est , 1996). Tw o of us re- cently surveyed aquatic habitats throughout the Pit River basin and discovered numerous previously unstudied pop - ulations ohi ssooidean snai ls ( Fr est & J ohannes, 1993a, b, 1994. 1995, 1996, 1997), including six spe cie s of Pyr- g ulopsis (Hydrobiidae), fom of which are new. The other two congener s that were collected previously had been considered restricted to the Klamath (P. arc hime di s) and northwestern Lahontan (P. er e mi co) basins. A new s pe - cies of Colli gy rus, a small amnicolid genus previously known only from the Snake River bas in and Great Basin of s outheas t Oregon (H er s hler , 1999), also wa s di scov - ered. All of this mat erial is described herein. In asmuch as s pecies of fres hwater ri ss ooidean snails often are dif - ficult to ascertain on the basis of morphological criteria alone , we have augmented our treatments with analyses of mitochondrial DNA se quences. Th ese analyses al so enabled evaluation of the phylog e ne tic relations hips of the sp eci es treated herein. In a subs equent pa per we will describe the numerous populations of another genus, Flu- min.ic olo , which were sampled during this s ur vey. PIT RIVER BASIN The Pit Ri ver basin occupie s about 17, 000 km 2 , cont ains a complex drainage (Pea se, 1965), and is largely si tu ated on the Modoc Plateau in La ss en, Modo c, Sh asta, a nd Si s- kiyou coun ties (Figure 1) . The mainstem Pit Ri ver or ig- inates near Alturas from the junction of two north-south streams, the North Fork and South Fork. Th ese fo rk s