WHITE] CRETACEOUS. 303 



covered in the Cretaceous strata of New Jersey, and pablislied by Dr. 

 Morton in liis Synopsis of the Cretaceous Formation of the United 

 States. It has since been recognized in widely separated localities iu 

 the United States and Mexico. Some of the varieties lose the promi- 

 nence of the beak of the lower valve, and approach an ordinary oyster 

 in appearance. A variety of this species, somewhat common in Texas, 

 is nan-ower than the typical form and has a more produced beak to the 

 lower valve. Conrad gave this form the name of O. navia, as already 

 mentioned. Figures of it are given on Plate XLIX. 



Gryphcea thirsce Gabb. 



Mr. Gabb described this as a Cretaceous species, but according to 

 Professor Heilprin it is found in true Tertiary strata. He regards it 

 as not generically distinct from the Ostrea, and it will be found on a 

 following page among the Eocene species under the name of Ostrea 

 thirsce. 



Oryphaa vesicularis Lamarck. 



(Plate XL VIII, Figs. 1, 2, 3, 4, 5.) 

 The species which is arranged under this name is a somewhat varia- 

 ble one. Dr. Morton published certain specimens of it under the name 

 of Ostrea mutabilis, and Say published others as Ostrea convexa. These 

 American forms are now generally regarded as specifically identical with 

 the long-known European species GryyliKa vesicularis of Lamarck. It 

 is found iu the Cretaceous rocks of the Atlantic and Gulf States. 



Oryphcea vomer Morton. 



Dr. Morton described this species as a Grypliwa, but I regard it as 

 not generically distinct from true Ostrea. (See remarks under Ostrea 

 vomer on a preceding page.) 



Genus Exogtka Say. 



Exogyra arietina Eoemer. 



(Plate LVT, Figs. 3, 4, 5.) 



This species is quite a common one in the Cretaceous rocks of Texas 

 and the adjacent parts of Mexico. It is generally known under the above 

 name, -which was published in Kreidebildungen von Texas, page 68, 

 Plate Vin, Figs. 10, a, 6, c, d, e, but it possibly ought to be called 

 Exogyra laxa Say. In 1826 Mr. Say described a shell under the name of 

 Delphinula laxa which, from its resemblance to certain specimens of E. 

 arietina, and my lack of knowledge of any other species to which it may 

 belong, I believe to have been a specimen of the larger valve of this 

 Exogyra. If I am correct in this supposition, Say's specimen was that of 

 a fossil instead of a recent shell, and an Exogyra instead of a Delphinula. 

 Say's very brief and unsatisfactory description of D. laxa may be found 



