22 Cincinnati Society of Natural History. 



minute clenticulations along the hingeline appear to occur. 

 The affinities of this species are beheved to be with Brachy- 

 prion. 



Geological position : In the upper part of the Crab 

 Orchard clay shales, between Poplar Flats and Martin's store, 

 in Lewis County, Kentucky, in the same slab with Calymene 

 cUntom, Beyrichia lata-triplicota, Choiietes vetusta, and 

 CainarotoccJiia congriicns. 



Stropheodonta (Brachyprion) planus, sp. nov. 

 Plate I, Figs. 13 A, B, C ; Plate II, Figs. 11 A, B. 



The largest specimen found so far has a length of 25 mm. 

 and a width of 31 mm. The convexity of the pedicel valve is 

 about 3 mm. The brachial valve is unknown, but is assumed 

 to be practically flat or slightly concave. The pedicel valve 

 is gently and evenly convex in small specimens, becoming 

 flatter near the anterior margin in larger specimens. The 

 postero-lateral angles vary from 80 to 90 degrees. The den- 

 tal lamellae are continued anteriorly as conspicuous ridges lim- 

 iting the postero-lateral boundaries of the muscular area. 

 Anteriorly, they disappear entirely, and here the boundaries 

 of the muscular area are uncertain. A distinct median low 

 septal ridge is present in all specimens. Usually it extends 

 as far from the cardinal margin as the dental ridges, or a 

 little farther. At the beak this median ridge is distinctly 

 wider. Radiating striae fine, about <S or 9 in a width of 2 mm., 

 subequal, except where other striae are intercalated, and here 

 they are alternately larger or smaller. 



West L^nion bed, at the blufif northwest of Martins, and 

 at Harin Hill, 4 miles west of Valley, in Lewis county, Ken- 

 tucky. Also at the same horizon in the quarry at the south- 

 eastern edge of West Union, Ohio. 



This species is distinguisl->ed from Stropheodonta orthi- 

 didea by the presence of the median septal ridge in the pedi- 

 cel valve, the less orthoid appearance of the muscular area 

 owing to the absence of any distinct anterior boundary, and 



22 



