Height 45 mm. 38 mm. 



Length of hinge line ? mm. 18 mm 



Convexity 3.5 mm. 3 mm. 



Remarks. This species of Posidonia has been found at only 

 two localities, and although it is fairly abundant, no perfect specimens 

 have been obtained, possibly on account of the thin, fragile character 

 of the shell; the small depressed anterior auricle is very seldom pre- 

 served. The species is characterized by its almost erect form, by its 

 small anterior ear, and by its strong regular concentric wrinkles. 

 There is no American species with which it can be compared except 

 P.pertenuis Beede; although similar to the latter species in contour, 

 the Ohio form can be distinguished by its much stronger, more numer- 

 ous, and more regular concentric wrinkles. It is the opinion of the 

 writer that the short auricle of both species is anterior, while the 

 longer is posterior. 1 



Horizon and locality. Lowellville horizon ? : black shale near 

 Holbein, Muskingum County (Locality 20), c. Lower Mercer, Rock 

 Hollow, Vinton County, a. Named in honor of the distinguished 

 paleontologist, Mr. George H. Girty, of the United States Geological 

 Survey. 



Genus Naiadites Dawson 



Naiadites elongata Dawson 

 PL IV, Figs. 22, 23, 24. 



1860 Naiadites elongata. Dawson, Supp. Acad. Geol., p. 43. 



Coal Measures: Nova Scotia. 

 1868 Naiadites elongata. Dawson, Acad. Geol., p. 204, Fig. 43. 



Middle Coal Measures: Joggins and Sidney, Nova Scotia. 



Description. This fresh or brackish water Pelecypod is char- 

 acterized by its elongate, oblique form, its small inconspicuous beak, 

 its short hinge line, and its fine, regular, concentric surface markings. 

 Its great variability of form is conspicuous, and it is possible that more 

 than one species may here be included under the name N. elongata. 

 However, it is generally found greatly crushed and distorted by pres- 

 sure so that a considerable variation in form is to be expected. It 

 occurs abundantly in the lower horizons of the Pottsville formation, 

 especially in the fissile shale and black band ore on the Bear Run 

 coal horizon in which it is the only fossil found with the exception of 

 plants and fish scales. A form of Naiadites which has provisionally 

 referred to the same species occurs very sparingly in the shales associ- 

 ated with the Quakertown or No. 2 coal. The Quakertown specimens 

 differ considerably in contour from those of the Bear Run horizon, 

 but again a possible explanation for these differences is distortion 

 due to pressure. Forms similar to those of the Bear Run member 



iBeede, J. W., Geo!. Surv. Kansas, Vol. VI, p. 136, pi. XIX, Fig. 5, 1900. Dr. Beede, in his des- 

 cription of this species, makes the longer auricle anterior in position. 



73 



