Helderberg Formation in ;Makvi,.vxd 29 



shells the concentric strias leave the plications nodulose at their crossing; 

 while there are specimens having the plications quite free from such char- 

 acters, and entirely smooth." Hall, 1852. 



This ubiquitous brachiopod is a common fossil of the Silurian and 

 Devoniaii systems in Maryland. 



Occurrence. — Helderberg Formation, Keysi:r Member. Devil's 

 Backbone, Tonoloway. Coeymans Member. Dawson, Devil's Backbone, 

 Maryland; Keyser, West Virginia. New Scotland Member. Devil's 

 Backbone, Miller's Spring near Cumberland. Ouiskany Formation, 

 EiDGELY Member. Cumberland. 



Collection. — Maryland Geological Survey. 



AtRYPA ? BICONVEXA n. sp. 

 Plate LXYIII, Figs. 1-3 



Description. — Shell subcircular in outline, wider than long; lateral, 

 cardinal, and anterior margins rounded. Valves of almost equal con- 

 vexity. Ventral valve convex, greatest convexity along the median portion 

 of the valve, the anterior portion abruptly deflected; beak incurved, 

 pointed. Dorsal valve convex, though not quite as convex in the median 

 portion as the ventral valve, the convexity being uniform, greatest con- 

 vexity about one-third the distance from the beak towards the anterior; 

 beak small, pointed, not prominent and only slightly incurved. The 

 anterior margin is abruptly inflected to correspond to the abrupt de- 

 flection of the anterior margin of the ventral valve. Surface marked by 

 eight simple rounded plications on each valve which radiate from the beak 

 and which are more prominent along and near the median portion of both 

 valves. In the ventral valve the three medial plications form an undefined 

 mesial elevation. There is, however, no indication of a sinus on the 

 brachial valve. Both valves are marked by well-defined concentric im- 

 bricating lamellae which give to the shell a distinctly rugose appearance. 



Dimensions : 13 mm. long and IG mm. wide. 



This shell most closely resembles A trypa rugosa of the Niagara. It is 

 not as convex as that species, the plications are not as numerous and do not 

 bifurcate, and it has no marked mesial depression on the dorsal valve. 



