LAMELLIBRANCHIATA OF THE LOWER MAELS. 41 



always. The very young shell, when of not more than an inch in length, is 

 often so sharply angular and so finely costate that it might well be consid- 

 ered as a distinct species. One example of an adult shell from Prairie Bluff, 

 Alabama, in the American Museum of Natural History, has grown attached to 

 some foreign flat substance over nearly the entire disk of the shell, leaving 

 the walls of the anterior and basal borders standing at right angles to the 

 atached surface. In the conditions of internal casts, a not unfrequent con- 

 dition of occurrence at many localities, it is more difficult still to recognize 

 its relations, especially so in casts of from one to two and a half inches in 

 length. In these conditions it is seen to 'be more elongated, and as the 

 inner surface of the flat valve, when young, is liable to be somewhat angu- 

 larly convex, it then assumes exactly the form and features of JEx. Texana 

 of Roemer from the Cretaceous beds in Texas. There is another form of 

 internal cast of a convex valve before me as I write, from the marl beds 

 at Holmdel, N. J., which, although highly convex, approaching gibbosity, 

 lacks that angularity usually seen on the shell itself. The beak of the spe- 

 cimen is broken, and the costse are few and extremely coarse, while the form 

 is somewhat spreading; and as it is accompanied by the cast of an oyster 

 on the opposite side of the specimen, was at first supposed to be a cast of a 

 very large individual of 0. panda Morton When, however, the marl was 

 cleared out from beneath the apical portion, the hinge structure of Exogyra 

 appeared distinctly. 



Mr. Gabb and Mr. Meek both refer Morton's Ostrea torosa (Synop., p. 52, 

 PI. X, Fig. 1 ) to this species. This reference I presume to be correct, simply 

 on their evidence, as I have not seen the specimen, although were I to judge 

 from the figure given by Dr. Morton I should have supposed it to have 

 been a Pinna. 



Formation and locality, — This species occurs more or less commonly at 

 nearly all the localities of the Lower Marl Beds of the Cretaceous in New 

 Jersey. It is also known at most of the localities of the formation in Del- 

 aware, Alabama, South Carolina, and Texas. Morton also cites it from 

 Tennessee and Arkansas, and Mr. Gabb gives it the range of '^all of the 

 United States except the northwest," where I believe it has never yet been 

 noticed. ' It is also found in Europe. 



