48 RESEARCHES UPON THE HYDROBIINM! 



It is interesting to notice tliat all the species of the genus as 

 yet described are severally reported to occur in mountainous dis- 

 tricts ; an instance of correlation of form to external conditions. 



Herrmansen suggests that the name should be changed to Pyr- 

 giscus, as the correct spelling. But Pyrgula is not a hybrid term, 

 since pyrgus is a Latin as M'ell as a Greek word. 



TRYONIA, Stm. 

 Tryonia, Stimpson, Am. Journ. of Conch., I, (1865), 54. 



Shell perforate, elongated, turreted, subulate, acute at summit 

 and rather pointed at base ; surface longitudinally ribbed or 

 plicated, not spinous ; whorls numerous, shouldered. Aperture 

 small, oblique, rhombo-ovate ; and somewhat pointed, sinuated, 

 and effuse at base ; outer lip thin and sharp, projecting below ; 

 inner lip appressed to the whorl above, peritreme however con- 

 tinuous. 



Operculum and lingual dentition unknown. 



Station, fresh water. 



Distribution, Southern California. 



Type T. claihrata, Stm. (Fig. 29). — Whorls eight. Longitudinal ribs 

 variable in number, usually about twelve to each 

 Fig. ^9. whorl. Surface otherwise smooth, or marked with 



delicate incremental strise. There is no trace of re- 

 volving striae or lines. Length, 0,2 inch. 



The specimens described are in a semi-fossilized con- 

 dition, mostly white, though not chalky, but with an 

 ivory-like hardness. Some of them are translucent, 

 looking as if silicified. From the circumstances under 

 which they were found, however, it is probable that 

 the species existed within a. very recent period, if not 

 indeed now living. 



* Large numbers of specimens were found, in company with other dead 

 fresh- water shells of the genera Physa, Planorhis, Amnicola, Sphaerium, etc., 

 in the basin of the Colorado Desert, Southern California, by Mr. Wm, P. 

 Blake, on one of the Pacific Railroad Surveys. The basin is the bed of an 

 ancient lake, now dry. The specimens collected by him are in the museum 

 of the Smithsonian Institution. 



The genus may be distinguished not only by the form and 

 sculpture of the shell, but by the shape of the aperture and the 



