56 CRETACEOUS GROUP 



costated, transversely corrugated, costae of the disc somewhat 

 dichotomous, sometimes fornicated ; apex lateral, with about 

 two volutions ; a single profound cicatrix ; hinge with two 

 nearly parallel, deeply excavated grooves, of which the inner 

 one is shortest and corrugated : upper valve flat, with nume- 

 rous elevated, concentric, squamous plates ; outer edge ab- 

 ruptly reflected from the inferior to the superior surface ; hinge 

 with a single groove on the edge. 



Var. A Smooth. 



Varies in size from an inch to ten inches in length. 



Abundant in almost all the arenaceous marls of this se- 

 ries. New Jersey, Delaware, South Carolina, Alabama, 

 Tennessee and Arkansas, possess numerous localities. 



Mr. Say is certainly correct in making a distinct ge- 

 nus of this fossil, in proof of which it will be found that 

 Mr. Sowerby has placed a congeneric shell with Chama, 

 (C. conica,) while M. Brongniart classes another with 

 Gryphsea, (G. auricularis.*) The true Exogyra has but 

 a single muscular impression in each valve, which suffi- 

 ciently distinguishes it from Chama, while it differs still 

 more strongly from Gryphsea. It is an interesting fact, 

 that all those European fossils which belong to the genus 

 Exogyra, have been found exclusively in acknowledged 

 secondary deposits. Thus the Chama conica and C. ha- 

 liotoidea, of Sowerby, are peculiar to the green sand of 

 England, while the Grypilea auricularis of Brongniart 

 has been found, in France, only in chalk marl.f 



* Geol. des Environs de Paris, pi. vi. fig. 9. 



t By reference to the " Mineral Conchology of Great Britain," (a work to 

 which I am under great obligations,) No. 104, it will be seen that Mr. Sowerby 

 has adopted Mr. Say's genus Exogyra, and transfers to it five fossil shells, (all 



