60 CRETACEOUS GROUP 

The annexed cut represents the most perfect specimen, 
of the natural size: but the largest measures, from beak 
to base, three inches; from anterior to posterior margins, 
two inches and a half, and was associated with numbers 
of nearly the same size in a mass of limestone. Mr. Con- 
rad observed a stratum of this species, in company with 
OstreA panda, near low water mark, in the interesting 
bluff at St. Stephens, on the Tombeckbe; and it is a 
characteristic species of the newer cretaceous deposits of 
the southern states. 
It has considerable general resemblance to P. spinosum 
of Sowerby ; but differs from the latter in having spines 
on each valve, those on P. spinosum being confined to the 
upper valve, as is beautifully illustrated in Cuvier and 
Brongniart’s Geologie des Environs de Paris, pl. iv; 
fig. 2. 
2. P. gregale, (S.G.M.) PI. v, fig. 6. 
Specific character. Suboval, thin, especially near the beaks ; 

