219 
Exogyrae appear confined principally to the Green- 
sand formation: but as we do not know that the American 
species Ex. costata has ever been found in Europe, it is 
going too far to consider that as a proof of the identity of 
the beds in which it occurs with our Green-sand. The 
smooth varieties of it which are said to resemble the 
G. haliotoidea of Min. Conch, we have not seen; but it 
probably is that species: if so, the consideration of it with 
Gryphaea convexa of Mr. Vanuxem and Dr. Morton, 
(which is G. globosa of Min. Conch, and Podopsis Gry- 
phaeoides of the French, and occurs in Green-sand as well 
as Chalk,) Baculites and other shells, will go far to prove 
what those gentlemen have suggested, that the beds in 
which these shells occur are the equivalents of the Green- 
sand and Chalk formations of Europe ; and we see no 
reason why different remote districts of the same forma- 
tion and period should not, like the present surface of the 
globe, be furnished with different shells, as well as vary 
in the proportions, arrangement and mineral characters, 
of its several members. 
EXOGYRA conica. 
TAB. DCV .—Jigs. 1. 2. & 3. 
Spec. Char. Smooth ; deep valve more or less 
gibbose, curved,, obtusely keeled along the 
middle; flat valve suborbicular, its beak im- 
pressed,, very much incurved. 
Syn. Chamae conica, recurvata, et plicata. Min. 
Conch, vol. 1 . p. 69, 70. t . 26. f. 2. 3. & 4. 
The short, almost orbicular flat valve and small size, 
rarely exceeding an inch and a half, are the principal 
marks by which to distinguish this from the following 
species. The nearly central keel will also assist. 
The small wings mentioned in the descriptions in vol. I. 
