93 [ Senate 



Eatonia eminens. 



Pal. N.Y. Vol. iii, pi. 37, f. 2. 



Shell somewhat depressed-globose or subquadri lateral, deeply 

 sinuate and abruptly elevated in front : ventral valve sloping 

 from the beak and sides into a broad undefined sinus, and ab- 

 ruptly extended in front with a regular curve into a large sub- 

 triangular prolongation, which lies nearly at right angles to the 

 plane of the valve near the beak : dorsal valve much the larger, 

 extremely elevated in front, and declining abruptly towards the 

 beak and sides ; mesial elevation with four plications, the two 

 middle ones much more prominent. ( The specimen a cast.) 



This species differs from the two preceding, to which it is related, in the 

 much greater prominence of the mesial fold in front, the plications broader 

 than in the first, and not nearly so many in the last. It also presents no- 

 table differences from C. medialis, in the characters of the internal im- 

 pressions. 



Geological position and locality. Lower Helderberg limestone, Ten- 

 nessee. - 



MeRISTA BELLA (n.S.). 



Pal. N.Y. Vol.iii, pl.40, f.l. 



Shell varying from subcircular or subquadrilateral to transversely 

 oval, usually somewhat broader than long, rather gibbous : 

 ventral valve a little the larger, most convex near the umbo ; 

 beak somewhat prominent and closely incurved : dorsal valve 

 convex ; both valves marked with a small sublinear mesial sinus, 

 that of the ventral valve stronger than the other, the two often 

 giving a distinct emarginate outline to the front. Surface smooth, 

 or marked by faint concentric lines of growth. 

 This species is characterized by its symmetrical form and the distinctly 

 emarginate character of the front, caused by the meeting of the small 

 mesial depressions of the two valves. The sinus on the front of the ventral 

 valve is always broader and deeper than that on the other, giving a waved 

 outline to the margins of the valves. Some of the specimens appear to have 

 a small open foramen in the point of the beak, but which may be acciden- 

 tal. 



