54 AMERICAN JOURNAL 



Lingual dentition of type : — Rhachidian tootli trapezoidal, 

 with the inferior or basal margin nearly straight, only faintly 

 trilobate ; cusp with nine denticles ; basal denticles minute, 

 four in number on each side, close to the lateral margins. In- 

 termediate tooth with the peduncle very long, three times as 

 long as the body and constricted at its juncture therewith; bod}- 

 subrhomboidal and excavated in the middle ; cusp with eleven 

 equal denticles. Lateral teeth constricted near the summits ; 

 inner lateral with fifteen denticles at the summit; outer lateral 

 with the summit broad, shaped like a chopping-knife, and 

 armed with twenty denticles. 



Station.- — -Fresh water. 



Distribution. — New Zealand. 



Type, P. COROLLA. 



Amnicola corolla Gould, U. S. Expl. Expedition, Moll.> 

 1852, p. 129, pi. ix., f. 149, a. c. 



TRYONIA. 



Shell perforate, elongated, turreted, subulate, acute at sum- 

 mit and rather pointed at base ; surface longitudinally ribbed 

 or plicated, not spinous; whorls numerous, shouldered ; aper- 

 ture small, oblique, rhombo-ovate, somewhat pointed, sinuated 

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

 inner lip appressed to the whorl above ; peritreme continuous. 



Station. — Fresh water. 



Distribution. — Southern California. 



Type, T. clatiirata, nov. sp., pi. 8, f. 1. 



Description. — Whorls eight. Longitudinal ribs variable in 

 number, usually about twelve to each Avhorl. Surface other- 

 wise smooth, or marked with delicate incremental striae. 

 There is no trace of revolving lines or striae. 



Leno'th 0'2 inch. 



The specimens described -are in a semi-fossilized condition, 

 mostly white, though not chalky, but with an ivory-like hard- 

 ness. Some of them are translucent, looking as if silicified. 

 From the circumstances under which they were found, how- 

 ever, it is probable tliat 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, in the Basin of the Colorado 

 Desert, by Mr. Wm. P. Blake. This basin is the bed of an 

 ancient lake, now dry. 



