﻿MHIHONIDITS 249 



sometimes angular in the young shell ; e])idermis rather smooth 

 and shining, yellowish to greenish, showing faint darker rays, 

 which are more or less broken and are made up of looped or 

 zigzag lines ; posterior slope quite strongly corrugated ; left 

 valve with two short, stumpy pseudocardinals and two delicate 

 laterals ; right valve with one pseudocardinal and one lateral ; 

 nacre purplish or greenish, often lurid and spotted. The male 

 shell is slightly arcuate and nearly straight on the base line ; 

 that of the female is somewhat more fully developed ventrally. 



Length (male) 40. height 22, diam. 13 mm. 



Length f female) 28, height 15, diam. 10 mm. 



Chattahoochee and Flint Rivers, Georgia. 



Type locality. Chattahoochee River, Columbus ; Atlanta ; 

 Flint River, Albany, (xcorgia. 

 lln'io penicillatus Lea, Pr. Ac. N. Sci. Phila., IX, 1857, p. 171 ; 



Jl. Ac. N. Sci. Phila., I^^ i85f). p. 203, pi. xxiii, fig. 85 ;: 



Obs., VIL 1859, p. 21. pi. xxiii, fig. 85. 

 Mar gar on (Unio) penicillatus Lka. Syn., 1870, p. 32. 

 Mcdionidus penicillatus Simtson. Syn., 1900, p. 590. 



This species has a rather shorter shell than any other mem- 

 ber of the group. It is smoother than conradicus and is less 

 arcuate and has a lower posterior ridge than acutissimus. 



Medionidus simpsonianus Walker. 



"Shell small, rather thin, somewhat inflated, elliptical, ine- 

 quilateral, strongly plicate on the posterior slope. Epidermis 

 dark yellow, smooth, polished, covered with dark green pen- 

 cilled rays, which tend to break into a network of angular 

 lines covering the entire surface. Anterior end compressed, 

 rounded, and slightly elevated above the line of the hinge 

 superiorly ; posterior extremity obtusely rounded, the tip nearly 

 on the median line of the shell ; posterior ridge somewhat 

 angled ; dorsal slope covered with strong, subconcentric, some- 

 what irregular ridges extending from the posterior ridge to the 

 margin ; basal margin regularly curved ; hinge margin nearly 

 straight, slightly angled between the cardinal and lateral teeth. 



