﻿NEW CRETACEOUS FOSSILS. 305 



the edge much more serrate in this than in the former species. While T. alceformis 

 is very elongated anteriorly, T. thoracica is obliquely truncated backwards. They 

 both have a small plate inside the shell, near the anterior extremity, but it is smaller 

 in T. thoracica. The anterior muscular impression in our species is rounded, quadri- 

 lateral, in Sowerby's it is crescentic, with the base up. 



This shell also differs from T. Emoryi, Con., in the latter being more inflated, in 

 having at least twenty-eight ribs, being proportionally more elongated anteriorly and 

 its having a much wider lunule, and the ribs on the lunule inclining first anteriorly 

 and then bending back, while in this species they have a backward inclination from 

 their origin. 



