﻿246 MKDioNinns 



sacs, which arc not so cUstinclly marked out as in JAVn[>silis. 

 but which have rounded bases ; mantle much thickened on its 

 lower edg-e. which is dark colored and sometimes |)ai)illous. 



Type, Uiiio conradiciis Lea. 



This ijenus contains a few rather closely related species 

 which are found in the Tennessee, Alabama and Chattahoochee 

 river systems and southeast to the Suwanee River, Florida. 

 It is closely related to Lampsilis, but the embryos are contained 

 in irregular, though distinctly marked, ovisacs near the center 

 of the outer gills instead of in regular ovisacs at the hinder 

 part of the gills. The marsupial swelling of the female shell 

 is less developed than in l.ampsilis and is placed farther for- 

 ward than in that genus. The male shell is g'enerally somewhat 

 arcuate and the species generally have a well-developed pos- 

 terior ridge, characters not often found in Lampsilis. 



Key to spf.ciks op Medionidus. 



Shell without a posterior ridge. M . conradicus. 



With a posterior ridge. 



Marked with undulating or broken rays. 



Solid. M. kingl 



Subsolid or thin. M. pan'ulus, penicillatus, 



simpsonianus, acutissimus. 

 Without rays. M. ivalkeri. 



Group of Medionidus conradicus. 



Shell small, usually with a well-detined posterior ridge ; pos- 

 terior end and sometimes the greater part of the shell slightly 

 wrinkled or nodulous ; epidermis rather smooth, painted with 

 rays broken into irregular arrow-head markings or blotches ; 

 pseudocardinals rather small and imperfectly developed ; lat- 

 erals of left valve separated by a narrow, shallow furrow ; 

 nacre greenish, purplish, or bluish. The male shell is often 

 decidedlv arcuate; that of the female is swollen at or behind 

 the center of the base. Animal having the character of the 

 uenus. 



