22 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 dated: 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 W. ovatus, Say, may be considered the type. It 
is more like UW. 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. p 
Unio Lamarcnianus. PI. XVII. Fig. 20. 
Testa nodulosa, subrotundd, compressa, in medio planulata, subequilaterali, postice subangulata, puncti- 
unculus passim radiata ; valvulis crassis ; natibus prominentibus, compressis, ad apices undulatis ; epidermide 
lutea, virido-radiatd; radiis numerosis interruplis; dentibus cardinalibus magnis, compressis; lateralibus 
brevibus crassisque; margarita 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 white. 
Hab. Caddo river, Arkansas, Professor Powell. Washitta river, near the Hot Springs, 
Arkansas, Dr. Hale. 
My cabinet and cabinets of Prof. Powell and Mr. Shaeffer. 
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 
distinct; posterior cicatrices distinct, dorsal cicatrices small and placed on the under side 
