DESCRIPTION OF SPECIES 187 



Description. Shell large, subquadrate in outline, convex, ratio of 

 length to width about 2:3; hinge line produced, always forming the 

 greatest width of the shell. Ventral valve strongly convex, beak 

 highly elevated and incurved over the hinge line; cardinal area, broad, 

 transversely striated; delthyrium large; mesial sinus well defined, 

 becoming broad and deep toward the front, always marked by at least 

 three strong, rounded ribs, but sometimes by five ribs, the lateral one 

 on each side in the latter case being smaller and formed by the bifurca- 

 tion of the ribs bounding the sinus. Dorsal valve less convex than 

 ventral, beak incurved and slightly elevated above the hinge line; 

 mesial fold strongly elevated, consisting of four to six ribs, the two 

 middle ones extending to the beak but bifurcating posterior to the 

 middle of the valve, so that four plications always reach the front; two 

 smaller, lateral plications sometimes originating by further bifurcation 

 of the outer ribs. Surface on either side of the fold and sinus marked 

 by ten to twelve simple, rounded ribs which are covered with fine, 

 radiating lines; crossed by a few faint concentric lines of growth near 

 the front and lateral margins. 



Dimensions. The following measurements were made of four large 

 specimens, two of which are ventral valves, and two, dorsal valves: 



Ventral Valves Dorsal Valves 



Length of hinge line (maximum 



width) 37mm. 33mm. 41mm. 35mm. 



Length of shell 20mm. 19mm. 28mm. 23mm. 



* Convexity 4.5mm. 4.5mm. 7mm. 5.5mm. 



Remarks. This species is common throughout the marine lime- 

 stones of the Pottsville formation of this State in and above the Boggs 

 member, but is especially abundant in the Boggs, Lower Mercer, and 

 McArthur limestones. It appears identical with the form from the 

 Humerosa formation of Colorado, described and figured by Girty as 

 S. boonensis? Both are characterized by their long hinge lines which 

 form the greatest width of the shell, and by their coarse ribs of which 

 three to five occupy the sinus and four to six the fold. In the Colorado 

 form there are twelve to thirteen lateral plications, while the form from 

 Ohio has ten to twelve on either side of the fold and sinus. There can 

 be little doubt that the forms under consideration belong to S. boonensis 

 as interpreted by Girty. 



Horizon and locality. Boggs member: Localities 26, 27, 28, 29, a. 

 Lower Mercer member: Scioto County, Locality 31, a; Jackson County 

 Localities 11, 33, c; Muskingum County, Locality 27, c; Licking County , 

 Locality 46, r; Mahoning County, Localities 54, 22, c. Upper Mercer 

 member: Scioto County, Locality 58, c; Muskingum County, Locality 

 38, c; Mahoning County, Locality 53, r. McArthur member: Jackson 

 County, Locality 80, c. Black Flint member: Jackson County, Locality 

 87, c; Vinton County, Locality 92, r. 



