THE MOLLUSCA 



1005 



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

 lateral teeth subobsolete. Subgenus Lasmigona Rafinesque. 



The type and only species, Sywphyuolii costala Raf. {Fig. 1501; X 5), is common in the St, 

 Lawrence and IMississippi draina^^c systems. 



Fig. 1501. 



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

 very heavy; lateral teeth imperfectly developed. 



Subgenus Plcrosystia Rafinesque. 



Only one species, Sy}nphy}wla complanata Bar. (Fi.i;. 1502; X?.), which has a wide range 

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



Fig. 1502. 



134 (139) Shell rhomboidal, inflated, with a well-developed posterior ridge; 

 pseudocardinals well developed; laterals subobsolete or 



wanting Alasmidonta Say 135 



Four subgenera: 135, 136, 137, 138. 



