ioi6 



FRESH-WATER BIOLOGY 



162 (163) Male and female shells much alike, but the latter is usually some- 

 what inflated in the post-basal region. Shell large, ovate, 

 usually rather thin, but in some species quite solid, gaping 

 at the anterior edge and on the dorsal slope; normally winged 

 on the dorsal slope, but the wing is often lost in the adult; 

 hinge complete. Marsupium occupying the posterior portion 

 of the outer gills. Glochidia celt-shaped, with two spines on 

 each valve and with gaping margins . . Proptera Rafinesque. 

 This group is well characterized by the large, usually thin shell, which is more or less alate in 



the dorsal region. It occurs throughout the St. Lawrence and Mississippi systems and extends 



south to Texas and Alabama. Type, P. alata Say (Fig. 1524; X i). 



FIG. 1524. 



