220 POTTSVILLE FAUNA OF OHIO 



Dimensions. A specimen of average size from the Lower Mercer 

 limestone measures: maximum length parallel to hinge line 16 mm., 

 length of hinge line 12 mm., height perpendicular to hinge line, near 

 posterior end 12 mm., height at beak 5 mm., length of umbonal ridge 

 19 mm. 



Horizon and locality. Of general distribution throughout the 

 Lower Mercer, Me Arthur, and Black Flint members, c. 



Genus Naiadites Dawson 



Naiadites elongata Dawson 

 PL XII, figs. 7-9 



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 character- 

 ized 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 pressure 

 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 been provisionally referred to 

 the same species, occurs very sparingly in the shales associated 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 have also been found 

 abundantly on the Sharon horizon, in the black shales averlying the 

 coal and in the ore, and also at one locality in the Boggs member. 



Dimensions. Average sized individuals from the various horizons 

 in which they occur have the following measurements: 



Sharon Quakertown Bear Run Boggs 



Length 15mm., 9mm. 15.5mm. 16.2mm. 18.5mm. 



Height 8.5 mm., 5.5 mm. 9.5mm. 9.7mm. 9.5mm. 



Thickness 3.8mm. 



Remarks. Members of this genus, as they are found in Ohio, oc- 

 cur either wholly by themselves, or associated with plant remains and 



