﻿AI.ASMIIXINTA 493 



the posterior border ; branchial opening papillose ; anal opening- 

 smooth or crenulate. 



Type, Monodonta unduiata Say. 



Say first called his genus Monodonca, but as that name was 

 preoccupied he changed it to Alasmidonta. Later on he wrote 

 the name Alasmodonta and this has been generally used by 

 conchologists. The shells are generally more inflated than 

 those of Symphynota, and the hinge of the right valve is not 

 cut into behind the pseudocardinals as it is in that genus. In 

 Alasmidonta the inner gills are united to the abdominal sac, 

 while in Symphynota they are not united to it, a character, 

 however, which is not to be greatly relied on. In the latter ge- 

 nus the marsupium is thick and pad-like and the mantle is 

 strongly attached at the pallial line. 



Key to species of Alasmidonta. 



Posterior slope of the shell corrugated. 



Shell solid, truncated behind, much inflated. A. marginata. 

 Shell subsolid, elongated. A. raveneliana 



Shell thin, rather short. A. varicosa. 



Posterior slope not corrugated. 

 Shell inflated. 



Solid with strong teeth, A. unduiata. 



Thin, greatly inflated, rough and dark. A. arcula. 



Moderately inflated, smooth. A. triangnlata. 



Shell scarcely inflated. 



Subsolid, dull, greenish, faintly rayed. A. minor; 



calceolus. 

 Dull brownish, rayless or feebly rayed. 



A. holstonia; georgiana. 

 Thin, sharply pointed behind. A. heterodon. 



Generally spinose. A. colUna 



Subgenus Alasmidonta s. s. 



Shell ovate-rhomboid, solid, inflated, shining, with very 

 strong, generally concentric, beak sculpture ; pseudocardinals 

 solid, stumpy, somewhat radiately ridged ; laterals short, very 

 imperfect, or wanting'; beak cavities deep, compressed. 



