350 Systematic Paleontology 



of the shell; 1he lateral and front luargins regularly rounded. The 

 pedicle valve iisuall}^ a little less convex than the opposite one; its beak 

 prominent, arched, but not strongly incurved; the sinus is rather abrupt, 

 not reaching quite to the beak. The surface of the brachial valve curves 

 gently to the margins, except toward the front, where the mesial fold is 

 rather abruptly elevated. The surface of each valve is marked by from 

 20-24 simple, angular plications, of which two or three, somewhat coarser 

 than the remainder, are depressed in the medial sinus, with a correspond- 

 ing number elevated in the fold of the brachial valve. 



" The dimensions of a rather large specimen are : Length 15 mm. ; 

 width 19.5 mm. ; thickness 10 mm. 



" This shell is a rather common one in the lower beds of the Decker 

 Ferry formation. It resembles Stenochisma formosa Hall, from the 

 higher portion of the Helderbergian series, but may be distinguished 

 from that species by its coarser plications, its greater proportional width 

 and by its less strongly convex valves, which gives to members of this 

 species a less thickness of shell." Weller, 1903. 



This species differs but little from 8. formosa of which it may be con- 

 sidered a variety. 



Occurrence. — Helderbeeg Foumation, Keyser Member. Cash Valley, 

 National Road east side of Warrior Mountain, Devil's Backbone, Corrigan- 

 ville, Maryland; Keyser, West Virginia; Hyndman, Pennsylvania. 



Collection. — Maryland Geological Survey. 



[Maynard.] 



Genus CAMAROTCECHIA Hall and Clarke' 



This rhynchonelloid genus is characterized internally by the fact that 

 the median septum of the brachial valve is divided posteriorly in such a 

 manner as to form an elongate cavity, which does not extend to the bottom 

 of the valve. Each branch of the septum supports one of tlie lateral divi- 

 sions of the hinge-plate, to which are attached the curved crural processes. 



'Hall and Clarke, 1893, Nat. Hist. N. Y., Pal., vol. viii, pt. ii, p. 189. 



