LAMELLIBEAKCHIATA FKOM THE EOCEl^E MAELS. 233 



arcuate, and subparallel with the cardinal margin, which is straight and 

 only a little shorter than the length of the shell behind the beaks ; posterior 

 end broad and squarely rounded, being nearly at right angles with the car- 

 dinal Hne. Umbonal ridge scarcely marked. Surface of the shell marked 

 by strong, radiating phcations, which are strongest on the anterior end and 

 faintest and more closely arranged on the postero-umbonal slope. On the 

 shell, or, as obtained by gutta-percha casts, in an outside imprint of the shell, 

 the surface of each rib is seen to be highly crested, and to be composed of 

 three ridges, a central or highest one and a smaller and more obscure one 

 on each side, with a moderately concave surface separating the different 

 sets. The ribs on the body of the shell and on the anterior end are often 

 covered along their crests with a series of nodes, formed by the concentric 

 lines of growth which thickly cover all parts of the shell. In the number 

 and strength of the ribs the specimens vary greatly. In the coarser forms 

 there are fifteen to twenty ribs, and on the finer ones twenty-five or more, 

 but they do not appear to be distinctive specifically, as there are all grades 

 between them. 



The species is very abundant, and forms the most common and most 

 prominent fossils of the beds in which it occurs. In its general form it is 

 perhaps as near C. rotundata of the Claiborne, Ala , beds as any American 

 species, but the ribs are coarser, and it differs in the surface markings and 

 in its more distinctly subquadrate outline. 



Formation and locality, — In the Green Marls at the top of the third bed, 

 associated with other Eocene fossils, at Farmingdale, Shark River, near 

 Monmouth, and other places. Mr. Gabb first described it from Mon- 

 mouth County, New Jersey, as (7. sithquadrata, but as Mr. Conrad had 

 previously given this name to a very different form from South Carolina, it 

 became necessary to change it, which Conrad did in his list of Eocene fos- 

 sils published in Vol. I, American Journal of Conchology. 



Cardita Brittoni, u. sp. 

 Plate XXX, Figs. 11 aud 12. 



Shell somewhat below a medium size, nearly circular in outline, but in 

 the condition of internal casts broadly oval transversely within the line of 



