valve is also compressed, some specimens showing a slightly convex 

 region surrounding the almost centrally located beak, the valve be- 

 coming concave toward the margins; a deep, broad slit extends from 

 the beak posteriorly to a point about two-thirds the distance from 

 the beak to the margin; slit widest in the central portion. The surface 

 is marked by numerous fine, concentric striae which are regularly 

 arranged; on a few forms very faint radiating markings are visible. 

 Crushed specimens from the Harrison ore, similar to those just des- 

 cribed, have been referred tentatively to the same species, while more 

 perfect forms from the Boggs member agree closely with McChesney's 

 description and are here identified with 0. capuliformis with more 

 confidence. 



Dimensions. The diameter of a small specimen is 7.5 mm.; 

 of a large specimen 18 mm. 



Remarks. The doubtful forms from the Harrison and Sharon 

 horizons may possibly be identical with 0. meekana Whitfield, which 

 is common and widely distributed in the higher Pottsville members 

 of this State, but the material at hand is too crushed to show specific 

 characters adequately. However, 0. meekana has the beak of the 

 dorsal valve eccentric, located about one-third of the length of the 

 shell from the posterior margin, while that of the specimens under 

 discussion is more central. 0. missouriensis Shumard is in general 

 a much smaller form, and also has the beak eccentric. 



Horizon and locality. Harrison ore: Jackson County (Locality 

 1), r; Sharon horizon : Jackson County (Locality 6), a. Boggs member: 

 near Hopewell P. 0., Muskingum County (Locality 26), r. 



Orbiculoidea missouriensis (Shumard) 



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

 Middle Coal Measures: Lexington and Charbonnier, Missouri. 



ShumarcTs description. "Shell circular; small; upper valve 

 depressed, conic, sloping gradually from the beak to the front, and 

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

 situated 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 depression at the bottom of which is an elliptical aperature. 

 The surface is marked with rather strong, concentric lines of increase 

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



Remarks. The small Orbiculoideas from the Lowellville and 

 Boggs members are entirely comparable with Shumard 's species in 

 respect to size, form, and eccentric beak, but differs in having the beak 

 slightly less depressed. A specimen of average size measures: diameter 

 6.5 mm., convexity 2.5 mm. 



48 



