﻿NEW CRETACEOUS FOSSILS. 327 



OPIS, Defrance. 



1. Opis bicarinata. A small orbiculato-trapezoidal, ventricose species, smooth ; 

 umbonal slope and posterior submargin carinated, postal area concave. 



2. Opis bella. Triangular, convex, polished, with regular and elegant concentric 

 lines, or minute flat ribs, separated by linear grooves ; posterior side cuneiform. 



About the size of the following species of Tellina, but a little longer and more 

 pointed posteriorly. 



TELLINA. 



Tellina ripleyana. Ovato-triangular, equilateral, compressed, polished ; concen- 

 trically lineated towards anterior extremity. Length nearly i inch ; height nearly 

 I inch. 



This and the two preceding species of Opis were discovered after the plates were 

 finished. They will be figured in a future number of this Journal. 



CRASSATELLA, Lam. 



Crasatella ripleyana. PI. 35, fig. 3. Triangular, very inequilateral, thick, 

 flexuous or plicate posteriorly ; umbonal slope angulated, subcarinated ; disk covered 

 with numerous irregular concentric furrows or coarse prominent lines ; posterior side 

 cuneiform, extremity obtusely rounded ; margin above obliquely truncated ; posterior 

 margin opposite the umbo angulated ; inferiorly concave ; margin densely crenu- 

 lated within. 



This, from the number of specimens sent, I judge to be an abundant species, very 

 distinct from all others of this country which have hitherto been described. The 

 cardinal plate is very broad, and the teeth thick and prominent. Its state of 

 preservation is equal to that of any Tertiary bivalve. 



NUCULA, Lam. 



Nucula percrassa. PI. 35, fig. 4. Triangular, very thick, ventricose, inequila- 

 teral, densely radiated with minute impressed lines ; anterior end subnasute, margin 

 obliquely truncated; posterior side cuneiform, extremity slightly salient, rounded, 

 situated much above the line of the base ; basal margin rounded, sometimes slightly 

 contracted anteriorly ; within crenulated on the margin ; submargin thickened ; 

 cicatrices deeply impressed ; cardinal teeth thick, fosset oblique. 



This species appears to be abundant and in excellent preservation; one specimen 

 has the valves united. The interior is silvery ; the pallial impression very distinct, 

 and an irregular line of five small cicatrices runs over the upper part of the valves in 

 the direction of the shell's length. The form of this Nucula is very unlike any other 

 American species. 



