DESCRIPTION OF SPECIES 217 



Posidonia sp. 



Description. Shell of median size, subcircular or a little ovate 

 in outline, slightly oblique, ratio of height to length about 13:12; 

 slightly convex in umbonal region, becoming flattened toward the mar- 

 gins; beaks almost centrally located, very small and inconspicuous, 

 depressed below hinge line; dorsal margin straight, about one-half as 

 long as the maximum diameter of the shell below, extremities rounded; 

 anterior and posterior margins broadly rounded and very slightly 

 flattened at the .middle of the shell, forming a continuous curve with 

 the ventral margin which is more narrowly rounded; anterior and 

 posterior auricles about equal in size. Surface marked by broad, ill- 

 defined, concentric wrinkles and by fine concentric lirae which cover 

 the entire shell. Shell, where preserved, thick. 



Dimensions. Height normal to dorsal margin 26 mm., length 

 normal to height 24 mm., convexity of single valve 1.5 mm.. 



Remarks. This rare form has been found only in the Lower Mercer 

 member of Muskingum and Licking counties, and is represented in the 

 collections studied by about six specimens, all more or less imperfect. 

 It seems identical with the species identified and figured by Miss Clara 

 G. Mark as Placunopsis carbonaria from the Lower Mercer limestone 

 in the Flint Ridge region; however, none of the specimens at hand 

 show the concentric wrinkles as strongly marked as the figured specimen, 

 Fig. 9, PL X, Bull. Den. Univ., Vol. XVI, 1911. Posidonia seems a 

 closer generic reference than the group of forms placed by Meek and 

 Worthen under Placunopsis. Concentric undulations of the surface 

 are of generic significance in Posidonia but do not ordinarily belong to 

 the other genus. Likewise, the total absence of radiating striae, even 

 though they are more or less evanescent, and of less importance the 

 lack of accessory superimposed markings, due to attachment to foreign 

 objects, which are characteristic of Placunopsis, but do not belong to 

 Posidonia, point to identification with the latter genus. The form 

 seems unlike any Pennsylvanian Posidonia in its specific relationships, 

 and probably represents an undescribed species. 



Horizon and locality. Lower Mercer limestone: Muskingum 

 County, Localities 28, 39, r; Licking County, Localities 47, 48, r. 



Genus Myalina DeKoninck 



Myalina pernaformis Cox 

 PI. XI, figs. 7-9 



1857 Myalina pernaformis. Cox, Geol. Kept. Ky., Vol. 3, p. 569, PL 8, Fig. 8. 

 Coal Measures: Providence, Hopkins County, Kentucky. 



