Maryland Geological Survey 393 



as far as the Chemung. In each of it? geological positions, however, it 

 presents peculiar characters and we are able to decide at once the geological 

 position of specimens by these peculiarities. On its first appearance in 

 the Clinton group, it shows its variable character in a remarkable degree, 

 and it is scarcely possible to avoid referring the individuals to distinct 

 species. In many of the young specimens the ventral valve is nearly flat, 

 or slightly convex, with a depression along the center from beak to base. 

 In specimens of medium size the valves are nearly equal and in older ones 

 the ventral valve is the more convex. Again there are others where, in the 

 young shell the ventral valve has no depression in the center, and is equally 

 convex with the dorsal valve. In the radiating striae or plications it is 

 equally variable; many specimens having them very distinctly dichot- 

 omous, while others are nearly undivided from the beak. In many young 

 shells the concentric stride 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 

 Devonian systems in Maryland. 



Occurrence. — Helderberg Formatiox, Keyser Member. Devil's 

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

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

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

 RiDGELY Member. Cumberland. 



Collection. — Maryland Geological Survey. 



[Maynard.] 



Atrypa ? biconvexa n. sp. 

 Plate LXVIII, Figs. 1-3 

 Description.— '&,\\q\\ 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 



