MARYLAND GEOLOGICAL SURVEY 279 



Description. " Shell very small, compressed, subcircular, approaching 

 subquadrangular ; height and breadth about equal; anterior margin short 

 and rounding or less rounded ; beaks elevated, nearer the anterior margin ; 

 dorsal margin sloping from the beaks, the anterior slope being the more 

 abrupt, and the margin behind the beaks straighter, more compressed and 

 sharper ; surface smooth ; internal casts showing the muscular impressions 

 to be comparatively rather distinct. Hinge unknown. Length, 0.06 inch ; 

 height slightly less; convexity, 0.03 inch." Meek, 1873. 



As indicated above, the hinge of this small pelecypod is unknown, as 

 no specimens have ever been found preserving the shell structure. Like 

 Cyclora minuta and other dwarfed gastropods and pelecypods, the 

 species occurs only as phosphatized casts or as molds in the rock. Such 

 casts of C. obliqua, however, occasionally show a denticulated margin 

 along the hinge line such as would be left by the denticles of a species of 

 Ctenodonta. 



This abundant small pelecypod was described from specimens found at 

 Cincinnati, Ohio, but it ranges in age from the Trenton to and through 

 the Richmond and occurs in many states. 



Occurrence. MARTINSBURG SHALE (Eden division). Jordans Knob, 

 one and one-half miles northeast of Fort Loudon, Pennsylvania. 



Collections. Maryland Geological Survey, U. S. National Museum. 



CTENODONTA FILISTRIATA Ulrich 

 Plate LIV, Figs. 26-29 



Ctenodonta filistriata Ulrich, 1894, Geol. Minnesota, vol. iii, pt. 2, p. 599, 

 figs. 44a-e. 



Description. This species has usually been identified with the Trenton 

 species Ctenodonta levata Hall, but it may be distinguished from this and 

 similar forms by the delicate, crowded, thread-like concentric lines which 

 cover the entire surface. Twelve to twenty of these lines may be counted 

 in a space 1 mm. wide. This surface ornamentation in connection with 

 the subovate forms of the shell and the rows of denticles on the hinge, 

 causes the species to be easily recognized. 



