2 h.VVKNPORT ACADEMY OF NATURAL SCIENi I 5. 



RISSOID^E. 



Genus Amnicola Gould & Haldeman (1841). 



Amnicola dalli, sp. nov. 



(Plate I., Figs. 4-6.) 



Amnicola dalli, Call.— Bull. U. S. Geol. Sur., No. 11, p. 45, Plate VI., Figs. 

 4-6 (1884). 



Shell narrowly umbilicate, obtusely conical, shining, slightly striated, 

 brown or greenish horn color; whorls four, convex, gradually increasing 

 in size; suture regularly impressed, somewhat deep; aperture rounded 

 before, somewhat angular behind, bluish white within ; lip simple, sharp, 

 margins joined by a thick callus, columella rather reflexed. 



Length, 3-5o mm ; breadth, 2.3o nim . 



Habitat and Station. — Mountain streams tributary to Pyramid Lake, 

 North-west Nevada. 



For the diagnosis of the lingual dentition I am indebted to Mr. 

 Charles E. Beecher, who has prepared the following description and 

 illustrations : 



"Jaw thin, membranaceous. 



"( kiontophore i.io mm long, . 13'™" wide. In a full-grown example 

 the odontophore has 94 transverse rows of teeth, with the formula 



3- 1 — 3- 



"Rhachidian tooth short and broad, with the inferior lateral angles 

 produced. Cusp with seven denticles, of which the central one is the 

 largest. The anterior lateral faces are each furnished with a short, 

 strong, conical denticle, and the adjacent lateral margin of the tooth 

 is thickened and slightly produced. Formula for rhachidian tooth : 



3 + 1 + 3 



1 + 1 



"Body of intermediate tooth quadrate; infero-interior angle some- 

 what produced ; furnished with a large bullation, into which the infero- 

 interior angle of the succeeding tooth appears to fit as if for articula- 

 tion. Peduncle long and straight. Cusp with seven strong angular 

 denticles, arranged according 10 the formula 2-J-1 + 4. 



"Body of the first lateral tooth elongate-triangular, oblique to the 

 direction of the broad peduncle. Cusp inflected, and carrying twenty- 

 three slender denticles. 



"Outer lateral tooth hamate, with no marked distinction between the 

 body and peduncle. Free extremity incurved and bearing thirty-four 

 minute denticles. The denticle formula is, therefore, 



