IOI2 



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 Ptychohranchus Simpson. 



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

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

 Arkansas. 



Fig. 1515. 



152 (153) Male 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- 

 Unes or maculations Dromus Simpson. 



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

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



Fig. 1516. 



153 (156) 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. ObovariaRa,^esq\ie . . 154 



