DESCRIPTION OF SPECIES 239 



1872 Pleurophorus oblongus. Meek, U. S. Geol. Surv. Nebraska, p. 212, PI. 10, 

 Figs. 4 a-c. 

 Upper Coal Measures: Nebraska City, Nebraska. 



Remarks. Pleurophorus oblongus is represented in the lower Potts- 

 ville formation by the internal mold of a single specimen from the 

 Sharon ore, and very sparingly in the middle and upper Ppttsville 

 formation. The individuals are for the most part well preserved so 

 that the identification could be made with considerable confidence. 

 The form from the Sharon ore is smaller than indicated by Meek's 

 description and figures; its dimensions are: length 8 mm., length of 

 hinge line 6 mm., maximum height near posterior end 4.5 mm., thick- 

 ness of right valve 1.2 mm. 



Horizon and locality. Sharon ore: Scioto County, Locality 2, r. 

 Lower Mercer member: Muskingum County, Locality 43, r; Licking 

 County, Localities 48, 49, r. McArthur member: Jackson County, 

 Locality 80, r; Vinton County, Locality 84, r. 



Pleurophorus spinulosa n. sp. 

 PI. XIV, figs. 9-11 



Description. This shell belongs to the same general type as the 

 one here described as Pleurophorus immaturus (P. subcostatus of Her- 

 rick) and in outline appears identical with that species as nearly as can 

 be observed from the somewhat imperfect specimens at hand. How- 

 ever the form under discussion differs from P. immaturus in the follow- 

 ing ways: (1) The shell is large, equaling almost twice the size of the 

 largest mature specimen of P. immaturus in the collections examined; 

 (2) only four radiating costae which are confined to the umbonal slope 

 were observed and these are more faintly expressed than those. of P. 

 immaturus, (3) the spinulose sculpture is coarser than is character- 

 istic of P. immaturus. 



Dimensions. The most perfect of the two known specimens of 

 this species measures: length 42+ mm., height 19 mm., convexity of 

 left valve 4.5 mm. 



Remarks. Some doubt exists as to whether this form is not 

 identical with P. immaturus, especially as it has been found at Bald 

 Knob, at the same horizon and comparatively close to Herrick's type 

 locality at Flint Ridge. However, the large size and the apparent 

 absence of a series of forms intergrading in size between them seems 

 to point to a distinct species. The coarser spinulose structure ought 

 naturally to accompany so great an increase in size, but the radiating 

 costae should also be correspondingly more numerous and more strongly 

 marked. Even after allowances are made for natural variations in tb.e 

 strength and number of costae, the discrepancies between shells of such 



