240 POTTSVILLE FAUNA OF OHIO 



different sizes seem too great to be found in the same species, and there- 

 fore these differences assume specific importance. The species, on the 

 other hand, is known from two not entirely perfect specimens, so that 

 later investigation may prove that the two forms are more closely re- 

 lated than is at present apparent. Both of the known specimens con- 

 sist of external molds, one of a right and the other of a left valve. 



Horizon and locality. Lower Mercer limestone: Licking County, 

 Bald Knob, Locality 46, r. 



Pleurophorus tropidophorus Meek 

 PI. XIII, figs. 20, 21 



1875 Pleurophorus tropidophorus. Meek, Pal. Ohio, Vol. II, p. 338, PI. 19, Figs. 10 a, b. 

 Coal Measures: Newark, Ohio. 



Meek's description. "Shell transversely oblong, much compressed, 

 with length a little greater than twice the height; posterior margin 

 flattened and bifurcated, the lower truncation being nearly vertical, 

 and the upper sloping obliquely downward and backward from the 

 hinder end of the hinge; cardinal margin straight, equaling about two- 

 thirds the length of the valves; anterior rounded below and sloping 

 abruptly forward from the beaks above; basal margin long, parallel to 

 the hinge, nearly straight for most of its length, or faintly sinuous near 

 the middle, rounding up anteriorly, and forming a more or less defined 

 angle at its connection with the lower part of the posterior margin 

 behind; posterior umbonal slope distinctly angular from the beaks to 

 the angular posterior basal extremity, while a second carina passes 

 obliquely backwards and downwards along the middle of the posterior 

 dorsal space above the umbonal ridge of each valve; beaks depressed 

 to the line of the cardinal margin, very little projecting, and placed one- 

 fifth to one-fourth the length of the valves from the anterior margin. 

 Surface marked by distinct concentric lines of growth, that become 

 strongly defined on the flanks and anterior parts of the valve, but are 

 less distinct on the space above and behind the umbonal angles." 



Dimensions. A specimen of typical size from the cannel coal 

 mine on Flint Ridge, Licking County, measures: length 29.5 mm.; 

 maximum height posterior to the beak 15 mm., convexity of a single 

 valve 4 mm. 



Remarks. This species is rare below the Lower Mercer limestone 

 but is fairly common and widely distributed in the latter and higher 

 members, especially in the Me Arthur limestone. The species shows 

 very little variation from place to place, and differs from Meek's fig- 

 ured specimen only in having the beaks slightly more depressed. In 

 his study of the fauna of Flint Ridge, Herrick 1 mentions that possibly 



derrick, C. L., Bull. Den. Univ., Vol. 2, p. 35, 1887. 



