236 POTTS VILLE FAUNA OF OHIO 



amined faint traces of a plication are visible on the flattened area be- 

 tween the umbonal ridge and the ridge bordering the escutcheon, which, 

 however, becomes obsolete toward the posterior margin. The very 

 faint radiating lines in the same region, mentioned by Price on the 

 West Virginia forms, are apparently absent, although they may not 

 have been preserved or may have been later destroyed. In other 

 respects the Ohio form agrees closely with the description and figures of 

 Price's species. 



Dimensions. The measurements of single valves of two individ- 

 uals are respectively: length 10.5 mm., 13,5 mm.; height 7 mm., 8.5 mm.; 

 convexity unknown. 



Horizon and locality. Lower Mercer black shale: Vinton County, 

 Rock Hollow, Locality 34, c. 



Genus Pleurophorus King 



Pleurophorus immaturus Herrick 

 PL XIV, figs. 5-8 



1887 Pleurophorus immaturus. Herrick, Bull. Den. Univ., Vol. 2 .p. 145, PL 14, 

 Fig. 17. (Also Pleurophorus subcostatus! Herrick, Idem., p. 35, PL 4, 

 Figs. 16, 16a.) 

 Coal Measures: Flint Ridge, Ohio. 



Description. Shell small, elongate-oblong in outline, with the 

 length slightly greater than twice the height; moderately convex in 

 umbonal region and along umbonal slope which extends obliquely 

 backward from -the beak to the posterior-inferior margin of the shell, 

 becoming somewhat depressed below the hinge line and along the 

 posterior extremity; cardinal and basal margins straight, converging a 

 little anteriorly, so that the anterior extremity is slightly but conspicu- 

 ously narrower than the posterior; beaks almost anterior in position, 

 small, not elevated above the cardinal margin; hinge line long, extending 

 from the beak about three-fourths the length of the shell; posterior 

 extremity obliquely truncated above and narrowly rounded below; 

 anterior extremity inconspicuous, slightly concave in outline below 

 beaks, then rounding abruptly into the basal margin. Surface marked 

 (1) by six to nine delicate but sharply defined lines or costae which 

 radiate posteriorly from the beak and which, when fully developed, 

 cover about four-fifths of the entire surface of the shell; (2) by very 

 fine, regular, subequal, concentric lines; and (3) by innumerable, minute 

 spinules which are larger and more conspicuous anteriorly than poster- 

 iorly where they are extremely small and crowded. These spinules are 

 both concentric and radial in arrangement and where most conspicuous 

 form tiny squares, each spinule marking the corner of a square. In 



