Maryland Geological Sukvey 231 



middle; posterior margin regularly rounded from the post-inferior ex- 

 tremity to the cardinal line which is short and rounded ; anterior end short 

 and narrow, regularly cun-ing from the lunule into the basal margin. 

 Valves regularly convex along the lower portion, becoming gibbous and 

 ventricose above and in the umbonal region ; beaks subanterior, prominent 

 and strongly incurved; the beak and umbo marked by a cincture, consist- 

 ing of a ridge and shallow furrows, which becomes somewhat obsolete on 

 the middle of the valve, but is continued to the basal margin, marking the 

 shell in its passage by an undulation in the striae which produces a slight 

 projection in the center, with a shallow constriction posterior to it and 

 these features alternate in the two valves. Surface marked by fine con- 

 centric striae, which become fasciculate toward the base and on the 

 anterior end, forming distinct ridges or undulations of the surface; no 

 radiating striae have been observed; the hinge has a strongly marked 

 ligamental area extending half the length of the shell, and a well defined 

 lunule. The species is readily distinguished by its subcircular outline, 

 and absence of a distinct cincture below the middle of the valve. 



A single, exfoliated, imperfect left valve from the B. & 0. R. R. cut at 

 21st Bridge is doubtfully referred to this species on account of its general 

 resemblance and marked subcircular outline. It shows fine concentric 

 striae and also distinct undulations which are much more conspicuous on 

 the anterior than the posterior part of the specimen. 



The outline is too imperfect to give correct proportions; but the speci- 

 men as preserved has a length of about 68 mm. and height of 65 mm. 



Occurrence. — Romney Formation, Hamilton Member. B. & 0. 

 E. R. cut at 21st Bridge; on Hancock-Harrisonville Road about 2 miles 

 north of Hancock. 



Collection. — Maryland Geological Survey. 



Genus EUTHYDESMA 



Euthydesma sp. 



Description. — An indeterminate species of the genus Euthydesma occurs 

 in the Onondaga fauna at Cumberland. 



