Maryland Geological Survey 301 



may bo several fine striae between the coarser ones. Internally the ventral 

 valve is not usually well preserved. The vascular area is small and not 

 usually well defined. The teeth which diverge from the beak, are 

 prominent, rounded, and become thicker towards their extremities. In 

 the dorsal valve the cardinal process is prominent and on both sides of it 

 are strong divergent dental lamellse which are produced forward and ex- 

 tend to at least the middle of the valve, converging toward the central 

 prominent ridge which extends from beneath the beak to the base, and 

 circumscribes a suboval muscular area. Dimensions of an average speci- 

 men : Length 6.5 mm. ; width 8 mm. 



Dalmanella clarki may be distinguished from Dahnanella postelegan- 

 tula of the Decker Ferry in being smaller while the sinus of the brachial 

 valve extends from the beak to the front. It differs from Dalmanellat 

 elegantula of the Clinton and jSTiagara in being wider than long, while the 

 brachial valve is more convex and the mesial sinus more conspicuous. It 

 differs from Dalmanella subcarinata in having the brachial valve more 

 convex, but not subcarinate. 



Occ«/-rence.--HELDERBERG Formation, Keyser Member. Cash 

 Valley. 



Collection. — ^Maryland Geological Survey. 



[Maynard.] 



Dalmanella concinna (Hall) 



Plate LIV, Figs. 11-13 



Orthis concinna Hall, 1859, Pal. N. Y., vol. iii, p. 172, pi. xa, figs. 1-3, 1861. 

 Dalmanella concinna Hall and Clarke, 1892, Pal. N. Y., vol. viii, pt. 1, pp. 

 207, 224. 



Description. — " Shell longitudinally semielliptical : valves unequally 

 convex: hinge-line straight, with the extremities subangular. Dorsal 

 valve convex, with a depression from beak to base, on each side of which 

 the shell is more convex, and thence sloping somewhat abruptly to the 

 sides : beak very small and scarcely incurved. Ventral valve very convex, 

 gibbous, and almost subcarinate in the middle: beak prominent, much 

 elevated above the hinge-line, and neatly incurved over the area. Area 

 comparatively large, the length greater than half the width of the shell. 



