330 Systematic Paleontology 



near the middle. Pedicle valve depressed, the umbonal region usually 

 irregularly concave ; the concave area, having been a facet of attachment 

 to some external object, retains the reverse of the contour of the object to 

 which the shell v\^as attached. Between the umbonal concavity and the 

 margin this valve is marked by one or more conspicuous, more or less 

 irregular, concentric wrinkles, with less conspicuous intennediate ones. 

 The cardinal area is unsymmetrical and variable because of the attach- 

 ment of the shell, but it is always rather low ; its margin is sometimes 

 sharply defined, but it is often very poorly defined. The brachial valve 

 is more or less regularly convex, the greatest convexity usually being 

 posterior to the middle; it is never so strongly marked by concentric 

 wrinkles as the opposite valve. Both valves marked by fine, more or less 

 unequal, radiating ribs, of which three or four occupy a space of 1 mm. at 

 the margin of the shell. The larger ribs extend the entire length of the 

 shell, while the shorter ones are intercalated at various distances from the 

 beak. 



" The dimensions of a rather large pedicle valve are : Length 30.5 mm. ; 

 width 25.5 mm. ; height of area at center 3 mm. ; maximum depth of valve 

 4.5 mm. A brachial valve having the same length and breadth has a con- 

 vexity of 9 mm. 



" This species differs greatly from any other Silurian species of the 

 genus, having a much closer resemblance to younger forms such as 0. 

 chemungensis of the Devonian and some Carboniferous species. It differs 

 from any of these younger forms, however, in its much larger facet of at- 

 tacliment upon the umbo of the pedicle valve." Weller, 1903. 



Most of the individuals referred to this species do not show the strong 

 wrinkles observed in the individuals from JSTew Jersey while the area is 

 higher at times. They also attain a larger size, a large individual being 

 27 mm. long and 31 mm. wide, convexity of brachial valve 10 mm. 



Occurrence. — Helderberg Formation, Keyser Member. Iveyser, 

 West Virginia; Pinto, Devil's Backbone, Market Street Bridge Cumber- 

 land, Cookerly, Cash Valley, 1| miles east of Push, Maryland. 



Collection. — Maryland Geological Survey. 



[Maynard.] 



