ioo6 



FRESH-WATER BIOLOGY 



135 (136) 



Shell ovate-rhomboid, solid, inflated; beak sculpture very coarse 

 and heavy; pseudocardinals large, solid; laterals very im- 

 perfect or wanting Subgenus Alasmidonta s.s. 



The type and only species, A. undulata 

 Say (Fig. 1503; X 1), is a characteristic shell 

 of the Atlantic states south to North Caro- 

 lina, but is not found west of central New 

 York. 



Fig. 1503. 



136 (137) Shell small, decidedly rhomboid; beak sculpture sUghtly corrugated; 

 teeth compressed Subgenus Pressodon Simpson. 



The typical species, Alasmidonta calceola Lea (Fig. 1504), 

 has a wide distribution through the Northern States from the 

 Mississippi eastward. Several other species occur in the 

 Atlantic and Southeastern States. One species, A . collina 

 Con., is remarkable for having one or more small spines 

 near the center of each valve. 



Fig. 1504. 



137 (138) Shell elongated, rhomboid, inflated, posterior slope sHghtly corru- 

 gated; pseudocardinals imperfect; laterals wanting. 



Subgenus Rugifera Simpson. 



The type, Alasmidonta marginata Say (Fig. 1505), ranges from New York and South Caro- 

 lina west to the Mississippi Valley. Another species is found only in the Tennessee and Cum- 

 berland river systems. 



Fig. 1505. 



