1000 FRESH-WATER BIOLOGY 



116 Shell large, irregularly oval, inflated; surface with a number of 



large, scattered tubercles. . . Section Plethobasus Simpson. 



This section contains only two species, inhabiting the Ohio and Tennessee drainage areas. 

 The type, Pleurobema aesophus Green (Fig. 1488; X I), extends west into Missouri and Minnesota. 



FIG. 1488. 



117 (108) Outer gills only serving as marsupia. Shell alike in both sexes; 



ovate to elongate, rounded in front, pointed or biangulate 

 behind; beaks nearer to the middle than to the anterior end; 

 hinge complete; surface usually smooth, dark brown to black, 

 of ten indistinctly rayed Unio Retzius . . 118 



118 (119, 120) Shell elongated, rhomboid or oval, more or less biangulated 



behind; surface smooth or feebly corrugated; beak sculpture 

 consisting of a few rather strong ridges, which are nearly 

 parallel to the growth lines or slightly double-looped. 



Section Elliptio Rafinesque. 



The typical section of this genus is restricted to the Old World. The section Elliptio is the 

 largest group of Unionidae represented in our fauna. More than ninety species are recognized. 

 The metropolis of the genus is in the Southeastern States, but representatives are found in all 

 of the Eastern, Southern, and Central States. Type, Unio crassidens Lam. (Fig. 1489; X 2). 



FIG. 1489. 



