New Species of Fresh Water Mollusk. 215 



acter which makes the young appear proportionately much 

 narrower than the adults; the anterior slope of the shell is 

 long and strongly convex, the posterior being short and 

 concave. 



The surface is marked by tine lines of growth. The largest 

 specimen which was secured has the following dimensions: 

 length, 3.5 mm.; width, i.S mm.; height, 1.5 mm. The 

 average dimensions are: length, 2.7 mm.; width, 1.7 mm.; 

 height, 1.2 mm. The radula is rather narrow, the dental for- 

 mula being 1 2-1- 12. A rachidian tooth and one of the inner- 

 most laterals will be figured on a plate in our next Bulletin. 



The animal is of a uniform bluish-white color; the foot is 

 ovate, wider anteriorly, and rather narrow for an Ancylus. 



This species occurs, not uncommonly, about five miles east 

 of Lincoln, Neb., in Dead Man's Run, a streamlet with 

 numerous pond-like enlargements. Specimens were found by 

 the author at all seasons of the year, adhering to shells of 

 Anodonia plana, sticks, leaves, etc. Though diligent search 

 has been made, no specimens have thus far been found in any 

 other than the above-mentioned locality. 



The only other specimen of the genus wdiich has been 

 found by the author in the vicinity of Lincoln is A. rivulan's. 

 Say, which is rather rare in Oak Creek, west of Lincoln. 



A more complete description and representation of the den- 

 tition will be made in a subsequent issue of this Bulletin, in 

 a paper on the comparative dentition of the species of Ancxliis. 



