TO GEOLOGY. 93 



its permanent characters. It is disposed to be cup-like, thin, 

 diaphanous and wrinkled, but this is not always the case. 

 In the two whole valves the beaks bend down immediately 

 from the margin and make no channel. 



The genus Ostrea is widely distributed through the foss- 

 iliferous strata. In Great Britain thirty-six species have 

 been observed from the Lias to the Crag. Lamarck has 

 described thirty-eight species from various beds and locali- 

 ties. Professor Sedge wick mentions one (species not 

 determined) in the Zechstein of Northumberland. We 

 have in M. Deshayes's Tertiary Tables seventy-two spe- 

 cies, forty-two of which are from the Paris basin (Eocene 

 period). 



In the Green Sand of New Jersey and Delaware, 

 Dr Morton has observed four or five species. From the 

 Tertiary of Maryland Mr Say has described the compressi- 

 rostra, and Mr Conrad has there observed the virginica 

 (Lamarck). Mr Conrad has described three species from 

 South Carolina and Alabama, and has noticed the virgi- 

 nica at Suffolk, Virginia. 



