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. spinosnm 

 of Sowerby ; but differs from the latter in having spines 

 on each valve, those on P. spinosnm being confined to the 

 upper valve, as is beautifully illustrated in Cuvier and 

 Brongniart's Geologic des Environs de Paris, pi. iv, 

 fig. 2. 



2. P. gregale, (S. G. M.) PI. v, f\^. 6. 



Specific character. Suboval, thin, especially near the beaks ; 



