2GG DESCRIPTION OF NEW 



Shell smooth, triangular, very much inflated, inequilateral, truncate behind; substance 

 of the shell thick, beaks large, inflated, rounded and placed towards the anterior margin ; 

 ligament short and very thick; epidermis dark-brown, polished, striate, in the young 

 nearly olive; lines of growth very distant; posterior slope flattened, with an obscure, 

 impressed, rather wide groove from the beaks to the margin ; umbonial slope angular 

 towards the beaks and inflated; cardinal teeth large, compressed, oblique, acuminate and 

 crenulate; lateral teeth long, lamellar, curved, enlarged at the posterior end and separated 

 from the cardinal tooth; anterior cicatrices distinct; posterior cicatrices confluent, dorsal 

 cicatrices placed on the under side and near the edge of the plate; palleal cicatrices not 

 deeply impressed; cavity of the shell very deep and rounded; cavity of the beaks very 

 deep and rounded ; nacre white and iridescent. 



Remarks. — Several specimens of this species have been in my possession for some 

 years. It belongs to the group of which U. ovatus, Say, may be considered the type. It 

 is more like U. occidens, (Nobis,) than ovatus, having the umbonial slope rounded. It 

 differs, however, from the latter species in having a more inflated disk, and in being darker 

 and rounder. It is not so inflated as capax, Green, but much like it. All the five indi- 

 viduals before me are females, unless the youngest one. The truncation of the posterior 

 margin is very remarkable. In the oldest specimen, that portion of the disk is so turned 

 in, as to present nearly a flat surface. The beaks are very much inflated and drawn 

 back. 



Unio Lamarckianus. PI. XVII. Fig. 20. 



Testa nodulosd, subrotunda, compressd, in medio planulatd, subequilaterali, poslice subangvlata, pvneti- 

 unculus passim radiatd; valvvlis crassis ; natibus prominent Urns, compressis, ad apices undulalis; cpidermide 

 luted, virido-radiatd ; radiis numerosis interruptis ; dentibus cardinalibus magnis, compressis ; lateralibus 

 brevibus crassisque; margaritd argented. 



Shell nodulose, subrotund, compressed, flattened in the middle, nearly equilateral, obtusely angular behind, 

 with dotted rays over the whole disk ; valves thick; beaks prominent, compressed, and angular at the point; 

 epidermis yellow, with green rays ; rays numerous and interrupted ; cardinal teeth large, compressed ; lateral 

 teeth short and thick ; nacre silvery while. 



llab. Caddo river, Arkansas, Professor Powell. Washitta river, near the Hot Springs, 

 Arkansas, Dr. Hale. 



My cabinet and cabinets of Prof. Powell and Mr. Shacffer. 

 Diam. .5, Length 1.1, Breadth 1.4 inches. 



Shell nodulous, subrotund, compressed, flattened from the beak to the basal margin, 

 nearly equilateral, obtusely angular behind, with dotted rays over the whole disk; sub- 

 stance of the shell thinner behind; beaks prominent, compressed and angular at the 

 point; ligament very short and thick; epidermis yellow, with green rays over the whole 

 disk, on the anterior part furnished with a few larger rays; lines of growth distant; rays 

 numerous, interrupted, dotted and chain-like; umbonial slope raised into an obtusely an- 

 gular ridge from the beaks to the posterior basal margin; cardinal teeth large, compressed, 

 crenulate; lateral teeth short, thick and enlarged at the posterior end; anterior cicatrices 

 distiiict; posterior cicatrices distinct, dorsal cicatrices small and placed on the under side 



