Maryland Geological Survey 371 



curved over it. Dorsal valve convex, more gibbous and consequently 

 more convex than the ventral valve, greatest convexity near the middle, 

 beak incurved under the beak of the ventral valve. Surface of each 

 valve contains indistinct, low, rounded, simple radiating plications, 

 whicli can only be seen from the center anteriorly and become more 

 conspicuous towards the front. From the center of the shell to the beak 

 the surface is smooth save for indistinct concentric lines of growth. An- 

 terior sk)pc abrupt; lateral margins abruptly deflected posteriorly and 

 rounded towards the anterior margin. Fold and sinus scarcely if at all 

 developed. 



Dimensions, 11 mm. long and 13.5 mm. wide. 



A single somewhat imperfect individual has been observed which has 

 been described above. It may be compared with W. glohosa from which 

 it differs in a number of respects. 



Occurrence. — Helderberg Formation? Keyser Member? Tonoloway. 



Collection. — Maryland Geological Survey. 



[Maynard.] 



Genus EATONIA Hall 



Eatonia siNGULARis (Vanuxem) 



Plate LXV, Figs. 21, 22 



Atrypa singularia Vanuxem, 1842, GeoL N. Y., Rept. Third Dist., p. 120, text 



fig. 3. 

 Eatonia singularia Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 243, pi. xxxviil, 



figs. 14-20, 1861. 



Description. — Valves " very unequal ; the upper one is flat, and forms 

 with tlio end opposite to the beak, nearly a right angle; the lower valve 

 curves, from the beak to the straight line on the opposite side. The middle 

 or mesial part is very much depressed." Vanuxem, 1842. 



" Shell wider than long, varying in form from ovate to transversely 

 elliptical or rhomboidal : hinge-line very slightly declining from the bealcs. 

 Ventral valve depressed convex in the middle towards the beak, and 

 concave between the center and the deflected margins; and below the 

 middle, extended into a deep broad sinus, which is prolonged and turned 

 upwards in fi'out at ridit angles to the lono-itudiual direction of the shell : 



