56 - CRETACEOUS GROUP 
costated, transversely corrugated, coste 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 
Gryphea, (G. auricularis.*) The true Lxogyra has but 
a single muscular impression in each valve, which suffi- 
ciently distinguishes it from Chama, while it differs still 
more strongly from Gryphea. 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 Cuama conica and C. ha- 
liotoidea, of Sowerby, are peculiar to the green sand of 
England, while the Grypuza auricularis of Brongniart 
has been found, in France, only in chalk marl.t 
* Geol. des Environs de Paris, pl. vi. fig. 9. 
+ 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: 
eet’ 
