172 POTTSVILLE FAUNA OP OHIO 



of points of difference can be noted which are believed by the writer to 

 be of specific importance and which seems to render WhitfiekTs species 

 a valid one. Both have the beak eccentric, situated about one-third 

 the diameter of the shell from the posterior margin, but 0. meekana is 

 a much larger form, adult specimens being at least twice the size of 

 those of the other species. The convexity of the dorsal valve is much 

 greater in 0. meekana than in 0. missouriensis, being in the former at 

 least one-half the diameter of the shell, and in the latter only one-third 

 the diameter of the shell. The surface sculpture of 0. missouriensis is 

 arranged in parabolic curves, while that of 0. meekana is circular. 

 0. convexa is also a considerably smaller form described from a far- 

 distant locality and from the Upper Pennsylvanian rocks, so that on 

 geographic and stratigraphic grounds, the identification with the Ohio 

 Pottsville form is unlikely. 0. meekana has been found rather com- 

 monly at Carbon Hill 1 and in the collections from Flint Ridge used for 

 this bulletin the type localities of Whitfield. 



Horizon and locality. Widely distributed throughout the Lower 

 Mercer and McArthur members, c. Upper Mercer member: Perry 

 County, Locality 63, c. Black Flint: Jackson County, Locality 89, c; 

 Vinton County, Localities 91, 92, c. 



Orbiculoidea missouriensis (Shumard) 



1858 Discina missouriensis. Shumard, Trans. St. Louis Acad. Sci., Vol. 1, p. 221. 

 Middle Coal Measures: Lexington and Charbonnier, Missouri. 



Shumard's description. "Shell circular; small; upper valve de- 

 pressed, conic, sloping gradually from the beak to the front, and rather 

 abruptly to the cardinal edge; beak rounded at tip, not curved, situ- 

 ated about one-third the diameter from the posterior edge, its elevation 

 equal to about one-third the diameter of the shell. Surface marked by 

 fine, close striae, which are arranged in concentric, nearly parabolic 

 curves, the extremities of which are directed to the front. Lower valve 

 circular, flat or slightly concave, having a large, deep, elliptical depres- 

 sion at the bottom of which is an elliptical aperture. The surface is 

 marked with rather strong, concentric lines of increase. Length of 

 average specimen, .33 (inches); height, .10." 



Remarks. Small Orbiculoideas from the Lowellville and Boggs 

 members are entirely comparable with Shumard's species in respect 

 to size, form, surface sculpture, and position of the beak, but differ in 

 having the beak slightly less depressed. A few individuals from the 

 Lower Mercer and McArthur members have also been referred to the 

 same species, although they differ in having the beak of the doisal 

 valve depressed so that the greatest convexity occurs a little anterior 



the stratigraphic position of Whitfield's fauna from Carbon Hill see footnote 

 on p. 155. 



