No. 109.] 91 



the plications arc nearly entirely obsolete, and the valves arc compressed 

 together around the front and lateral margins. 



Geological position and localitrj. Shaly limestone of the Lower Ilelder- 

 herg, Albany and Schoharie counties. 



Eatonia sinuata. 



Pal. X.Y. Vol. iii, pi. 101 A, f. 2. 



Shell circular or longitudinally oval : ventral valve concave, 

 except in tlie unibonial region, from which point, as well as 

 from the lateral margins, it slopes generally into the broad deep 

 sinus without defined margins : dorsal valve convex, rising in 

 front into a broad undefined mesial prominence, often nearly as 

 high as the highest part of the central region of the valve ; beak 

 incurved. Surface marked by thirty-six to forty strong elevated 

 rounded or subangular plications on each valve. Visceral im- 

 pressions large, broad, and marked with radiating plications 

 towards the margin, strongly defined by an elevated border : 

 impression of the adductor muscles cardiform, small, located in 

 the middle of the visceral impression, longitudinally striate. 



The two middle plications on the dorsal valve are separated 

 by a wider depression than between those on other parts of the 

 shell, which continues quite up to the beak : in this depression 

 there is sometimes near the front a slender plication, which 

 becomes obsolete before reaching the beak. 



The surface of this shell was doubtless also marked by fine concentric 

 lines of growth, but none of the specimens coming under my observation 

 are in a condition to have preserved them. 



This species differs from C. vicdialis in being proportionally more elon- 

 gate, having more plications, and abroad^ and less distinctly defined sinus 

 in the ventral valve. The two plications bordering the sinus and mesial fold 

 of the former species arc also proportionally much broader than in this one. 



Geolfigical position ajid locality. Oriskany sandstone, Cumberland, Md. 



