﻿UNITED STATES. 207 



Bab. — Chatahoochee Elver, at Roswell, Cobb County, Georgia. N. A. Pratt, Jr., 

 and J. Postell. 



My cabinet and cabinets of Mr. Pratt, Mr. Postell and Dr. Lewis. 

 Diam. '-5, Length '9, Breadth 1*3 inches. 



Shell smooth, elliptical, rather inflated, obliquely rounded behind and regularly 

 rounded before, straightened at the base, inequilateral ; substance of the shell rather 

 thin, thicker before; beaks somewhat prominent,' with four or five large undulations 

 at the tip ; ligament short, thin and light brown ; epidermis very deep olive brown, 

 with very regular dark green rays over the whole disk ; cardinal teeth rather small, 

 crenulate, oblique and double in both valves ; lateral teeth rather long and nearly 

 straight ; anterior cicatrices distinct and well impressed ; posterior cicatrices confluent . 

 and lightly impressed ; dorsal cicatrices in an oblique line above the cavity of the 

 beaks ; cavity of the shell rather deep and rounded ; cavity of the beaks rather deep 

 and subangular ; nacre white and very iridescent. 



Soft parts. — PI. 24, fig. 88 a. Branchial uterus situated towards the posterior end 

 of the outer leaf, and consists of five or six sacks on each. These are distended, and 

 protrude a little below the edge of the branchial leaf. Branchial large, nearly 

 semicircular, the inner ones much the larger. The two inner ones become united for 

 one-fifth of an inch on the inferior border, towards the posterior end and immediately 

 under the branchial uteri of the outer leaves. This union continues to the upper 

 edges of the branchia, and therefore forms a blind sack before and behind the 

 junction. Posteriorly to this junction of the two inner leaves of the branchial, the 

 two outer ones become united on each side to the inner ones, and remain so united to 

 the posterior end.* The inner branchial are united to the abdominal sack the whole 

 length, except at the point or very edge of the posterior retractor muscle of the foot. 

 Palpi rather small, thickish, triangular, united only a short distance down the 

 posterior edges. Mantle thin, dirty white, thick on the margin, having papilla on 

 the margin below the branchial opening. Branchial opening rather large, with 

 numerous, small, brownish papillae. Anal opening small, with minute brownish 

 papillae. Super-anal opening rather large, brownish within and united below. Color 

 of the mass dirty white. 



Embryonic shell elongate pouch-shape. See pi. 5, fig. 8. 



Remarhs. — It is greatly to be regretted that a single specimen only of this species 

 should have been received with the soft parts. I was struck at once, on opening it, 

 with the singular construction of the juncture of the branchial leaves, which the 

 artist has attempted to represent in the plate. It is so entirely different from 

 anything I have ever seen in the examination of the immense numbers of this family 



* If the peculiar anatomical structure, here described be found to be persistent, theu it must be considered, 

 iu any system founded on the structure of the soft parts, to be a distinct genus. 



53 



