172 UNITED STATES GEOLOGICAL SURVEY Of THE TERRITORIES. 



of valves more or less crenate within posteriorly, and smooth forward; hinge- 

 teeth usually strong, and not differing from some sections of Cardium ; pallial 

 line with a small sinus. 



This group includes three sections, that may be defined as follows: 



1. protocardia, Beyrich (typical). 



Shell thick, usually gibbous, or cordate-subglobose, with length 

 generally nearly or quite equaling the height; hinge strong; surface- 

 sculpturing usually well defined. — (Type as already stated.) 



2. pachycardium, Conrad. 



Shell very massive, much higher than long; valves very gibbous; 

 hinge remarkably strong; surface-sculpturing rather obscure, the 

 posterior radiating costa? being nearly obsolete. — Cardium (Lcevi- 

 cardium) Spillmani, Conrad. 



3. LEPTOCARDIA, Meek. 



(fir). Shell small, very thin, about as long as high; hinge weak; 

 surface nearly smooth, the posterior radiating costse being obsolete, or 

 often only indicated by crenulations along the posterior third of the 

 free margins within ; pallial line with two shallow sinuses. — Cardium 

 subquadratum, Evans and Shumard. 



(5). Shell higher than long, or agreeing more nearly in form with 

 Pachycardium, but very thin. — Cardium pert enue, M. & II. 



Mr. Conrad mentions some differences in the relative sizes and forms 

 of the hinge-teeth of Pachycardium as distinguishing characters from Proto- 

 cardia; but the species of the latter group vary in this respect so much, that 

 I should think no great importance can be attached to these differences. In 

 proposing the genus, he named the species cited above as the type, but 

 also included the East Indian Cretaceous species Cardium bisectum, Forbes. 

 Dr. Stoliczka, who has had an opportunity to study good specimens of the 

 latter species, says that it agrees so exactly in all respects with Protocardia, 

 that he does not think Mr. Conrad's proposed genus should be retained even 

 as a subgenus. If may be that Mr. Conrad was a little unfortunate in 

 including that shell; but his typical species contrasts so very strongly in 

 form, with the usual outline of nearly all the well-marked species of Proto- 

 cardia, that it seems to be entitled to be regarded a! least as the type of a 

 distinct subgenus. 



