260 



Annals of the Carnegie Museum. 



Possibly the color of the soft parts and of the eggs (placentae) is also 

 characteristic. (See below.) 



This genus greatly resembles in shell-structure the genus Quadrula, 

 chiefly the pustulosa-group and the genus Rotundaria. But in the 

 marsupium it is more advanced than the former, and in the color of 

 the soft parts and the eggs it suggests affinity with Fusconaja. It 

 surely is a connecting link between the more primitive Unionincc and 

 those of the type of the genus Pleurobema. Simpson has recognized 

 the peculiar character of P. asopus, and I think his section Plctlw- 

 basus is entitled to generic rank. 



Plethobasus aesopus (Green). 

 Some fifty specimens have been examined, chiefly from the Allegheny 

 River in Armstrong Co., Pennsylvania. Among them were gravid 

 females, and the latter were found exclusively in the month of July. 

 The species is typically tachytictic. 



an 



>br 



o = mp 



Fig. 8. Plethobasus asopus (Green). Sterile female (just discharged), from 

 Allegheny River, Kelly, Armstrong Co., Pa. (Cam. Mus., No. 61, 4,598.) Coll. 

 July 25, 1910. 



Anal and supra-anal separated by a very short mantle-connection, 

 which was found missing in a small number of cases. Branchial 

 opening with strong papillae, anal with very fine papillae. Palpi of 

 usual shape, their posterior margins connected for about one-third 

 of their length. 



Gills long and broad, the inner the wider. Anterior attachment as 

 usual. Diaphragm normal. Inner lamina of inner gills free. Septa 



