10 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



Operculum ovate, thin, corneous, spiral, with polar point well forward 

 and approximating the^columella. 



Jaw thin, membranaceous. 



Odontophore with teeth arranged in transverse rows, according to 

 the formula 3 + 1 + 3. Formula for denticles of rhachidian : 



1 + 1. 



Distribution : Western and South-western United States, in fresh or 

 brackish water. 



So far as known to us, the typical European Pyrgula are bicarinate 

 or multicarinate. The type of the genus is the species described by 

 Michelin* as Melania helvetica. The founders of the genus, Christoforo 

 and Jan, described the same form as Pyrgula annulata, from a locality 

 in Switzerland. Figures n and 12 of Plate II. are drawn from Switzer- 

 land specimens of this form. 



As above defined, this genus will include the form described by Mr. 

 John Wolf as Pyrgula s c alar if or mis. ^ Although the first described 

 species, it was not considered advisable to constitute this form the type 

 of the genus. Being a post-pliocene fossil, it was impossible to indi- 

 cate those characters in the animal itself which are desirable in framing 

 an intelligible diagnosis. These have, so far as the opercukim and 

 dentition go, been studied in the Nevada form only. The remaining 

 species to be included are, besides the type, P. mississippiensis, sp. 

 nov., and P. spinosus, sp. nov. 



DESCRIPTIONS OF THE SPECIES OF PYRGULOPSIS. 



Pyrgulopsis nevadensis Stearns. 



(Plate II., Fig-s. i-io.) 



Pyrgula nevadensis Stearns. Proc. Phila. Acad. Nat. Sci., p. 173, figure (1883). 

 Call and Beecher, Am. Nat., Sept. 1884, Vol. XVIII. , pp. 851-855; from 

 this paper the present account of the dentition is copied. Call, Bull. U. S. Geol. 

 Survey, No. n, 1884. 



Shell small, somewhat elongated, variable, turreted, imperforate; 

 whorls 4/^-5^2, strongly unicarinate on periphery, otherwise smooth; 

 epidermis shining, light straw color or whitish, white at suture; suture 

 deeply and regularly impressed, made conspicuous by the approximat- 

 ing carina; aperture very oblique, roundly ovate, with an angle on 

 outer edge corresponding to the excavated carina, posteriorly sharply 



*Ma#azin de Zoologie, p. 37, Plate xxxvii. (1831.) 



\Vide American Journal of Conchology, Vol. V., 1869, p. 198, Plate xvii., Fig". 3. 



