SHUMARD—PERMIAN FOSSILS. 293, 
having a deep narrow sinus omen: from beak to front, 
sides age eardinal margin mate o about on ne-half the 
width 
e 
wider than long. Dorsal valve 
convex, gibbous in full-grown specimens; beak s 
-and slightly passing th border ; area nar- 
row, its margin gentty arcuate, Surface e¢ 
d, irregular, radiating, usually trif ribs, which are indis- 
fro on the lateral i ey are separated © 
margins; the 
ws, and the number on the “boner amounts to from 18 
ie of the 
