214 POTTS VILLE FAUNA OF OHIO 



unornamented object. The surface of the valve in question is marked 

 by fine concentric lines and by very faint concentric wrinkles. In view 

 of the surface sculpture, the most plausible specific reference seems to 

 be Cox's Avicula acosta, although our form has the hinge line slightly 

 less produced and the lateral margins just below less concave. If the 

 species is the same as that described as A. acosta, the valve must be 

 the opposite (right ?) to Cox's figured specimen, which therefore is 

 apparently equivalve and lacks a byssal sinus in either valve. If it is 

 not the same as A. acosta, the presence of a byssal notch on the opposite 

 valve of either specimen cannot be determined. 



Assuming a specific identity with A. acosta, our form seems gen- 

 erically most closely allied with Posidonia, although it might almost as 

 well be placed with Plagiostoma which also lacks a byssal notch in 

 either valve, especially as the concentric wrinkles are very faint and 

 poorly expressed, markings of generic importance in the genus Posi- 

 donia. Two other specimens of the same valve and probably repre- 

 senting the same form, one also from the Lower Mercer and the other 

 from the Black Flint member, resemble Cox's species more closely in 

 outline, but have a surface sculpture similar to the previously de- 

 scribed individual. 



Dimensions. Height 10 mm., length of hinge line 9 mm., maxi- 

 mum width below hinge line 10.5 mm. 



Horizon and locality. Lower Mercer limestone : Muskingum 

 County, Localities 43, 44, r. Black Flint: Vinton County, Locality 91, r. 



Posidonia girtyi n. sp. 

 PL XII, figs. 1-4 



Description. Shell large, ovate, compressed, slightly oblique; 

 beaks prominent, almost anterior, slightly elevated above and incurved 

 over the hinge line; greatest convexity in umbonal region, posterior and 

 ventral portions flattened; hinge line straight, about three-fifths the 

 length of the shell below; anterior margin almost straight or slightly 

 convex, with small anterior auricle; ventral and posterior margins con- 

 tinuously rounded, with the latter truncated above and forming an 

 angle of about 70 degrees with the hinge line; posterior auricle large, 

 flattened. Surface marked by numerous strong, regular, concentric 

 folds, separated by broad, rounded furrows. Shell very thin and fragile. 



Dimensions. The dimensions of two cotypes are: 



1 2 



Length '_ 40mm.? 32mm. 



Height 45mm. 38mm.? 



Length of hinge line 20mm.? 18mm. 



Convexity 3.5 mm. 3 mm. 



