260 ANNALS OF THE CARNEGIE MUSEUM. 
Possibly the color of the soft parts and of the eggs (placentz) 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 Unionine and 
those of the type of the genus Pleurobema. Simpson has recognized 
the peculiar character of P. @sopus, and I think his section Pletho- 
basus is entitled to generic rank. 
Plethobasus zsopus (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. 
OoO=mp 
Fic. 8. Plethobasus @sopus (Green). Sterile female (just discharged), from 
Allegheny River, Kelly, Armstrong Co., Pa. (Carn. 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 papilla, anal with very fine papilla. 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 
