216 POTTSVILLE FAUNA OP OHIO 



Posidonia vintonensis n. sp. 

 PL XII, figs. 5, 6 



Description. Shell small, obliquely semicircular to semiovate, 

 moderately convex in umbonal region and along umbonal slope, but 

 depressed along basal and posterior margins and also on large posterior 

 auricle; axis of shell forming an angle of about 45 degrees with the 

 hinge line; beak small and inconspicuous, scarcely elevated above the 

 hinge line, located about one-fourth the length of the shell from the 

 anterior extremity. Hinge line straight, long; basal and posterior 

 margins forming a continuous curve, the latter meeting the hinge line 

 at an obtuse, rounded angle; anterior margin oblique, almost straight, 

 with small but conspicuous anterior auricle. Surface marked by 

 rather coarse, sharply defined, concentric wrinkles which become obso- 

 lete or nearly so on the large, flattened posterior auricle. Shell ex- 

 tremely thin and fragile. 



Dimensions. The dimensions of two cotypes, the first of average 

 size and the second of unusually large size, are: 



1 2 



Length at hinge line 9mm. 15mm. 



Maximum height perpendicular to hinge line 7 mm. 12 mm. 



Length along oblique axis 9mm. 14mm. 



Remarks. This small species occurs abundantly in the black shales 

 on the Lower Mercer horizon in Rock Hollow, one and one-half miles 

 south of McArthur, Vinton County. It is here associated with the 

 large species of Posidonia, described in the present report as P. girtyi, 

 although no specimens were discovered in association with the latter 

 species in the ravine near Holbein, Muskingum County. The known 

 occurrence is confined to the single locality in Vinton County. The 

 species is characterized by its small size, very oblique form, small al- 

 though prominent anterior auricle, and strong concentric undulations. 

 The extremely fragile, depressed posterior auricle and posterior margin 

 are very seldom found in a perfect state of preservation. 



Generically this form has been placed with Posidonia rather than 

 with Posidoniella on account of its less anterior beak and its conspicu- 

 ous anterior auricle. Pteria is another closely related genus, which, 

 however, lacks the strongly wrinkled concentric surface sculpture of 

 Posidonia. Although our form is also similar to Naiadites, its surface 

 markings and its association with marine fauna serve to separate it 

 from that genus. 



Horizon and locality. Black shale on Lower Mercer horizon: 

 Rock Hollow, Elk Township, Vinton County, Locality 34, a. 



