The Nautilus. 



Vol. v. may, 1891. No. 1. 



DESCRIPTION OF TWO NEW SPECIES OF UNIO. FROM ARKANSAS. 



BY WM. A. MARSH. 



TJnio Pilsbryi. 



Shell quadrate, somewhat oblique, plicate, striate, slightly inflated 

 before, compressed behind, very inequilateral, obtusely angular 

 before, sub-biangular posteriorly, valves thick before, thin behind, 

 beaks small and flat. Epidermis dark reddish-brown, without rays. 

 Cardinal teeth compressed, oblique, grooved, double in both valves, 

 lateral teeth long, slightly curved, solid, anterior cicatrices deep, 

 pit-like, granular, posterior cicatrices not very distinct, confluent, 

 nacre dull white. 



Habitat. Little Red River, Arkansas. 



Remarks. This species bears some resemblance to U. perplicatus 

 Con., which is abundant in the same stream, but differs in being 

 much flatter, beaks very much smaller, the growth lines much closer ; 

 it also differs in being coarsely striate over the entire surface of the 

 shell ; in its undulations it is very different, the plications being very 

 small, and much l)roken up; in fact, semi-nodulous; from U. 

 undulatus Barnes, which is also abundant in the same stream, it 

 differs entirely in outlines, in the manner and disposition of its folds, 

 color of epidermis, nacre, teeth, etc., etc. 



I name this shell in honor of Mr. H. A Pilsbry, Conservator 

 Conchological Section, Academy of Natural Sciences, Phila., who 

 first called my attention to its specific value. Specimens may be 



