IIkI.DKIJBEKG FoHMATIOX IX 3.Ia1!Y1.A\D 11 



Occurrence. — Helderbekg Formation, Keyser Member. Devil's 

 Backbone, Viaduct Cimiberland, Cash Valley, Touoloway, Maryland; 

 Keyser, West A'irginia. 



Collection. — Maryland Geological Survey. 



Genus SCHUCHERTELLA Girty 

 SCHUCHERTELLA DECKERENSIS (Wellcr) 



riate LX, Figs. 13-16 



Orthothetes deckerensis Weller, 1903, Geol. Survey N. J., Pal., vol. iii, pp. 

 229-230, pi. XX, figs. 6-7. 



Description. — " Shell subplauo-convcx, transversely subelliptical in out- 

 line, the hinge-line shorter than the greatest width of the shell, which is 

 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 was 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 intermediate 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 20.5 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 



