218 POTTS VILLE FAUNA OF OHIO 



Description. This form is not rare in Pottsville rocks of Ohio, 

 but is particularly common and characteristic of the Boggs and Black 

 Flint members. In size it varies within wide limits from very large 

 specimens to immature ones which are about one-half the size of that 

 figured by Cox. The following measurements of individuals selected 

 from the Boggs and Black Fint members will illustrate this variation 

 in size: 



Black 



Length of greatest diameter, Boggs Flint 



from beak to posterior-infer- / % 



i or margin 22mm. 48mm. 60mm. 69mm. 



Width, normal to umbonal ridge 10 mm. 21 mm. 30.5 mm. 35.5 mm. 



Length of hinge line 12 mm. 21 mm. 28+ mm. 44 mm. 



Convexity of left valve 3mm. 6mm. 10mm. crushed 



Anterior margin straight, forming an angle of 60 to 65 degrees with 

 the hinge line, sub parallel to posterior margin; posterior margin slightly 

 convex in outline, rounding broadly into the front below the middle of 

 the shell. Sometimes the posterior end of the hinge line is somewhat 

 produced with the posterior-lateral margin directly below slightly con- 

 stricted, so that the posterior-superior angle forms a small wing-like 

 extension; sometimes the posterior margin meets the hinge line at 

 right angles, while more commonly the hinge line is short and the 

 posterior-superior angle is truncated and slightly rounded. The sur- 

 face is marked by strong, regular, concentric lines of growth. 



Remarks. The form resembles that of M. kansasensis Shumard, 

 but lacks the fluted lamellae of that species. It differs from M. re- 

 curvirostris Meek and Worthen in its more oblique form and in lacking 

 the prominent, strongly recurved beak which characterizes that species. 



Horizon and locality. L Boggs member: Muskingum County, Lo- 

 calities 26, 27, 28, c. Lower Mercer limestone: Perry County, Locality 

 35, r; Muskingum County, Locality 45, c; Licking County, Locality 46, c. 

 Upper Mercer -member: Muskingum County, Localities 28, 68, r. 

 McArthur limestone: widely distributed, c. Black Flint: Vinton 

 County, Localities 91, 92, r. 



Myalina pernaformis Cox var. 

 PL XI, figs. 5, 6 



Description. Two small specimens of Myalina from the Harrison 

 ore of Jackson County have been classed as a variety of Myalina perna- 

 formis. The axis is somewhat more oblique and the size much less 

 than that of mature specimens of M. pernaformis from the higher 

 Pottsville horizons. The angle between the cardinal and anterior 

 margins measures about 52 degrees. It can be compared to M. cunei- 



