6o Bryant Walke;r 



Subgenus PLATYNAIAS Walker, 1917. 



Shell smooth, subsolid, shining, rayed; beak sculpture sharply double- 

 looped ; teeth deHcate ; laterals compressed, moderately developed. 



Fig. 176 

 Type: Symphynota compressa Lea, fig. 176. 



Subgenus LASMIGONA s. s. 



Shell subrhomboid, compressed, corrugated behind; beaks low, their 

 sculpture consisting of several coarse ridges, which generally fall into two 

 slight loops, and often with radiating ridges in front and behind ; epidermis 



Fig. 177 



shining; laterals partly developed, consisting of blurred ridges, which slope 

 diagonally downward and backward on the hinge-plate ; cavities of the 

 beaks shallow. 



Type: Alasmidonta costata Raf., fig. i77- 



