499 PAL^OfTTOLOGY OF NEW-YORK. 



Eatonia sinuata. 



Plate CI A. Fio. 3-6. 

 Ealonia nntuUa : Hall, Regents' Report for 1856, p. 91; Palsaozoic Fossils, 1857, p. 51. 



Shell circular or longitudinally oval. Ventral valve concave, except in 

 the umbonial region, whence, as well as from the lateral margins, it 

 slopes gradually into the broad deep sinus without defined margins. 

 Dorsal valve convex, rising from the middle towards the front into a 

 broad undefined mesial prominence, which is 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 sub- 

 angular plications on each valve. 



Muscular impressions large, broad, and marked with radiating plications 

 towards the margin, strongly defined by an elevated border : impres- 

 sion of the adductor muscles cordiform, small, located in the middle 

 of the large muscular area. 

 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 the shell was doubtless also marked by fine concentric 



lines of growth, but none of the specimens which have come under my 



observation are in a condition to have preserved them. 



This species differs from E. medialis in being proportionally more elongate, having 



more plications, and a broader and less distinctly defined sinus in the ventral valve. 



The two plications bordering the sinus and mesial fold of the former species are also 



proportionally much broader than in this one. 



Fig. 3 a. Dorsal view of a large individual, which shows the prevailing form. 



Fig. 3 6, c. Profile and front views of the same. 



Fig. 4. Dorsal view of a more orbicular form. 



Fig. 5. Ventral view of a cast, showing the strong muscular impression with the small 

 central adductor impression. 



Fig. 6. Ventral view of a cast preserving a part of the shell ; the form of the shell and 

 muscular impression more rotund than the preceding. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



