188 POTTS VILLE FAUNA OF OHIO 



Spirifer cameratus Morton 

 PL IX, figs. 11, 12 



1836 Spirifer cameratus. Morton, Am. Jour. Sci., 1st Ser., Vol. 29, p. 150, PI. 2, 

 Fig. 3. 

 Goal Measures: Ohio Valley. 



Description. Large Spirifers of the cameratus type constitute one 

 of the most common and characteristic forms not only of the middle 

 and upper Pottsville, but of almost the entire Pennsylvanian system 

 of Ohio. It has not been found to occur, however, .below the Lower 

 Mercer limestone. The type is distinguished by its long hinge line, and 

 by its strong, numerous, freely bifurcating plications which are arranged 

 in bundles of three, five, or occasionally seven. The forms in the col- 

 lections studied show considerable variation in the length of the hinge 

 line, the number and size of the plications, and the tendency toward 

 bifurcation, and it is believed that several varieties doubtlessly are 

 present. As all gradations exist in the above-mentioned respects 

 between opposite extremes, no plausible basis for further subdivision 

 of the species was discovered. The more coarsely plicate type, how- 

 ever, greatly predominates in Pottsville rocks of this State. 



Dimensions. A specimen of average size from the Lower Mercer 

 limestone measures: length 30 mm., width at hinge line 38 mm., con- 

 vexity of ventral valve 10 mm. 



Horizon and locality. Widely distributed throughout the Lower 

 Mercer, Upper Mercer, McArthur, and Black Flint members, a. 



Spirifer opimus Hall 

 PI. IX, figs 13-20 



1858 Spirifer opimus. Hall, Geol. Iowa, Vol. 1, Pt. 2, p. 711, PI. 28, Figs, la, b. 



Coal Measures: Ohio, Maryland, Iowa, etc. 

 1915 Spirifer opimus. Mather, Bull. Den. Univ., Vol. XVIII, p. 185, PI. 12, Figs. 7-7c. 



Morrow formation: Arkansas and Oklahoma. 



Hall's description. "Shell rotund, gibbous, length and width 

 nearly equal; hinge line equaling or sometimes a little greater or less 

 than the width of the shell below: valves nearly equally gibbous in 

 their greatest convexity. Dorsal valve regularly convex, with a strong 

 well-defined mesial fold which is simple at the apex, dividing a little 

 below, and each division again dicotomizing, the two middle divisions 

 stronger than the lateral ones, and separated by a well-defined groove; 

 in some of the smaller shells, the lateral plications of the mesial fold are 

 feebly or not at all developed: beak elevated a little above the hinge- 

 line, and incurving over a narrow defined area. Ventral valve most 



