54 CRETACEOUS GROUP 
Var. A. Upper valve subconvex, umbo depressed. 
This interesting fossil expands laterally from the hinge 
on each side, but most on that which is lobed. The ra- 
diated lines on the upper valve are few in number and 
irregular, but in most instances strongly marked. The 
lower valve has a ridge or line extending from the hinge 
round the shell, and parallel to the edge: at this point 
the reflected margin of the upper valve commences, and 
is continued obliquely upwards to the disc of the corres- 
ponding valve. Some individuals are attached by the 
beak, like Popopsis gryphzxoides. Some varieties are 
so like Ostrea vesicularis, Lam., as scarcely to be dis- 
tinguished from it. ‘The O. vestcularis is characteristic 
of the European chalk. Others approximate to GryPHEA 
bullata, Sowerby, pl. ccclxviii. Faujas de St. Fond 
figures a shell very similar to ours from the plateau of 
Maestricht. 
The protean forms of the two preceding species, have 
puzzled most naturalists who have examined thems and 
although I have at different times possessed more than an 
hundred individuals, I not unfrequently meet with inter- 
mediate varieties that may as readily be referred to one 
species as the other. It is even possible that they all be- 
long to a single species. 
Accompanies the preceding species, and is found as 
far west as Tennessee and Missouri. 
3. G. vomer, (S.G. M.) Pl. ix. fig. 5. 
Specific character. Oblong-oval, thin, narrow; lower valve 
deep, longitudinally curved : beak prominent, curved laterally ; 
‘ 


