MARYLAND GEOLOGICAL SURVEY 365 



broadly and evenly curved though having the usual backward swing. 

 Ventral edge of carapace obtuse, scarcely flattened, with a slight furrow 

 on each side near the edge of the right valve in which a row of minute 

 punctae is generally distinguishable; overlap extending all around the 

 free edges, strongest ventrally ; except in rare instances, neither valve has 

 a flange or flattened border, and when present it is in all cases very 

 narrow and undefined ; dorsal edge somewhat thickened, especially upon 

 the left side. Surface of the valves smooth or very faintly pitted, rather 

 evenly convex with the greatest thickness somewhat beneath the center; 

 a low ridge-like thickening along the posterior half of the dorsal margin 

 of the left valve is to be noticed. Eye tubercle just distinguishable in 

 most cases, rarely so distinct as in the specimen figured, often not to be 

 detected. On the inner surface, however, it is always marked by a dis- 

 tinct pit. Muscle spot not distinguishable externally except when the 

 specimens are weathered, but on the inner side it is often well marked 

 and surrounded by fine reticulating radial lines, short dorsally, longest 

 post-ventrally. On the inner side of the ventral edge of the right valve 

 there are two rows of small papillae, three to five in each, the number 

 seeming to increase with age. The purpose of these papillae, one series 

 of which occurs in the anterior third, the other in the posterior evidently 

 was to prevent undue overlapping of the valves by presenting an obstacle 

 to the entering ventral edge of the left valve." Ulrich, 1894. 



Although this species has been cited as a wide-spread characteristic 

 fossil of both the Stones Elver and Black Eiver groups, the typical form 

 is really restricted to the latter rocks. The original types were described 

 from, the Platteville limestone of Wisconsin, the equivalent of the Low- 

 ville limestone of more eastern localities. 



Occurrence. -CHAMBERSBTJRG LIMESTONE (Tetradium cellulosum bed) . 

 Fort London and other localities in Franklin County, Pennsylvania. 

 STONES EIVER LIMESTONE. Pinesburg and Wilson, Maryland. 



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



