54 CRETACEOUS GROUP 



Far. 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 Podopsis gryphseoides. Some varieties are 

 so like Ostrea vesicularis, Lam., as scarcely to be dis- 

 tinguished from it. The O. vesicularis is characteristic 

 of the European chalk. Others approximate to Grypilka 

 bullata, Sowerby, pi. 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 them ; 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.) PI. ix. fig. 5. 

 Specific character. Oblong-oval, thin, narrow ; lower valve 

 deep, longitudinally curved; beak prominent, curved laterally; 



