954 ZOOLOGY MOLLUSCA. 



are crossed by revolving ribs. In H. idahoensis, Newcomb (ibid., ii, 1866, 

 1), we find the surface raised into sharp ribs, parallel with the lines of 

 accretion ; and as in all other respects this species does not appear to differ 

 from Strigosa, it is very probable that idahoensis will also prove to be a 

 variety of this protean species. 



" Included in the collection are two specimens of the genus Tryonia, 

 Stimpson. This curious little genus was heretofore considered restricted to 

 the Colorado desert of Southern California, where, in a fossilized condition, 

 it exists in such numbers as frequently to cover the surface of the ground. 

 Two species have been described, viz : the type T. (Melania) exigua, Conrad 

 (of which Melania protect, Gould, is a synonym), and Tryonia datlirata, 

 Stimpson. The two Utah specimens are probably T. exigua. The genus 

 Tryonia is included in a group of very small species of Amnicolidcs, having 

 the whorls ornamented by ribs, nodules, or spines ; it includes the genera 



" Paludestrina, Orb., 1841, West India and South America. 



" Pyrgula, Crist, and Jan., 1832, Europe.* 



" Potamopyrgus, Stimp., 1865, New Zealand. 



" In addition to the above, the collection includes Succinea lineata, W. G-. 

 Binney ; Limnfea pahistris, Miill. ; L. stagnalis, Linn., and L. desidiosa, Say ; 

 Physa eUiptica, Lea ; and Planorbis trivolvis, Say ; the latter very large 

 specimen, with the margin of the aperture expanded, like those from the 

 Saint Lawrence River described by Mr. Whiteaves as P. macrostonms." 



*Mr. Johu Wolf has described Pyrgula scalariformis from the Post Pliocene 

 near Tazwell, Illinois Kiver. Other minute species may be detected hereafter, when 

 our rivers are more carefully explored, as in France a number of new species have 

 rewarded the minute search of recent collectors. 



