SIIUMARD—TERTIARY AND CRETACEOUS FOssiILs. 123 
long, angulated. from er eee Gane i fom 
excavated superiorly, becoming nearly plane below. 
ing almost a ie angle with the umbonial 1 
impression and its edges strongly defi ed; oo 
cordate, ce oa onger than wide, deeply impressed, and 
its edges strongly defined; beaks rounded, elevated incurved, 
situated nearest the anterior CRE: ; surface marked with 
subimbricating ng = fine bey > ribs attenuated in front 
sion is pat ae en broad ovate and distinct; the posterior 
one is shallow subovate, broadly rounded below, narrow and 
truncated above; palleal i pan broad and distinctly im- 
pressed ; sinus triangular, not deep. 
en the exterior crust of the shell is removed, we find 
re sosasitne radiating ribs extending from ing : base, —— = 
y very — srenget oe concentric-waved lines, an 
