THE MOLLUSCA 



lOO: 



132 (131, 133) Shell subrhomboid, compressed, posterior slope corrugated; 

 lateral teeth subobsolete. Subgenus Lasmigona Rafinesque. 



The type and only species, Symphynota costata Raf. (Fig, 1501; X i), is common in the St. 

 Lawrence and Mississippi drainage systems. 



133 (131, 132) Shell large, ovate-rhomboid, subcompressed, smooth; hinge 

 very heavy; lateral teeth imperfectly developed. 



Subgenus Pterosygna Rafinesque. 



Only one species, Symphynota complanata Bar. (Fig. 1502; Xi), which has a wide range 

 from the Great Lakes and the upper Mississippi south into Alabama and Arkansas. 



Fig. 1502 



134 (139) Shell rhomboidal, inflated, with a wcll-dcvclopcd posterior ndge; 

 pseudocardinals well developed; laterals subobsolete or 



wanting Alasmidonta Say . . 13 5 



Four subgenera: 135, 136, 137, 138. 



