DESCRIPTION OF SPECIES 215 



Remarks. This species of Posidonia has been found at only two 

 localities, one in Vinton and the other in Muskingum County > and 

 although it is fairly abundant, no entirely perfect specimens have been 

 obtained, probably on account of the thin, fragile character of the shell. 

 The small, depressed anterior auricle is very seldom preserved. The 

 species is characterized by its almost erect form, by its small anterior 

 ear, and by its strong, regular concentric wrinkles. On large mature 

 specimens these wrinkles are coarser and more irregular in arrangement 

 in the umbonal region, becoming finer and more regular in the basal 

 half toward the posterior margin of the shell; young, immature shells, 

 on the other hand, exhibit only the coarser concentric wrinkles of the 

 umbonal region of mature specimens. On a few of the larger forms 

 very faint indications of coarse, radiating plications were noted in the 

 furrows between the folds. There is no American species of Posidonia 

 with which our form can be compared except P. (Posidonomya) per- 

 tenuis Beede; although very similar to the latter in contour, the Ohio 

 species can be distinguished by its stronger, more numerous and more 

 regular concentric wrinkles, and possibly by its smaller anterior auricle. 

 It seems probable that the short auricle of P. pertenuis as well as that 

 of the species under discussion is anterior, while the longer is posterior. 1 



In determining the generic position of this form three genera 

 deserve serious consideration: Posidonia ( =Posidonomya), Posidoniella, 

 and Caneyella, all of which are characterized surficially by coarse con- 

 centric folds. Posidonia, rather than Posidoniella, seems correct on 

 account of the presence of the small anterior auricle which is present 

 on the former, but which is absent on the latter species; likewise, the 

 beaks are anterior but not terminal as is generally the case in Posi- 

 doniella. It differs from Posidonomya beecheri Bronn, the type of the 

 genus, principally in its more erect form and smaller anterior auricle. 

 The form under discussion also seems very close to Caneyella, which 

 may show an anterior auricle, and it strongly resembles C. vaughani 

 Girty in its generic relationships. However, as the Ohio specimens 

 are too poorly preserved to determine with any degree of certainty the 

 presence of the large conspicuous byssal aperture between the valves 

 which characterizes the genus Caneyella, it is deemed advisable to place 

 our form under the genus Posidonia rather than under Caneyella. 



Horizon and locality. Black shale on Lowellville horizon ?: 

 ravine near Holbein, Muskingum County, Locality 20, c. Black 

 Shale on I^ower Mercer horizon: Rock Hollow, Elk Township, Vinton 

 County, Locality 34, a. Named in honor of the distinguished paleon- 

 tologist of the United States Geological Survey, Dr. George H. Girty. 



'Beede, J. W., Geol. Surv. Kansas, Vol. VI, p. 136, PI. XIX, Fig. 5, 1900. 

 Dr. Beede orients his species so that the longer auricle is anterior in position. 



