1012 



FRESH-WATER BIOLOGY 



151 (152) Male and female shells alike. Whole outer gill serving as marsu- 

 pium, its edge thrown into a number of folds. Shell elon- 

 gate-triangular, solid and thick; hinge complete; hinge plate 

 wide and flat. . . ... . . . Ptychobranchus Simpson. 



The typical species, P.phaseolus Hild. (Fig. 1515; XI), is common, ranging from Michigan 

 south to Alabama and Louisiana. Five other species are known, four in Alabama and one in 

 Arkansas. 



FIG. 1515. 



i5 2 (iS3) Ma l e and female shells alike. Marsupium occupying nearly the 

 whole of the outer gill and, when fully developed, folded. 

 Shell solid, round- triangular; hinge complete; hinge plate 

 wide and flat; surface sculptured by irregular ridges and 

 humps, painted with undulating, radiating, broken hair- 

 lines or maculations Dromus Simpson. 



Only two species are known, both from the Tennessee and Cumberland river systems. Type, 

 D. dramas Lea (Fig. 1516; Xi). 



FIG. 1516. 



*53 ( J 56) Male and female shells different, the latter being slightly inflated 

 in the post-basal region. Shell short oval, rounded, or retuse. 

 Marsupium occupying the posterior portion of the outer 

 gills and projecting far below the rest of the branchiae, 

 dolabriform or kidney-shaped. Obovaria Rafinesque . . 154 



