338 PALEONTOLOGY OF OHIO. 



straight, but rounding up behind, so as to connect with the hinge at 

 right angles; dorsal margin very nearly straight, equaling the greatest 

 length of the valves, and provided with a narrow marginal ridge; beaks 

 very nearly obsolete, extremely oblique, and placed. a little behind the 

 narrow, obtusely pointed, anterior extremity. Surface with numerous 

 concentric lines and lamellae running parallel to the basal and posterior 

 margins. 



Length, 2.03 inches ; height, 0.62 inch. 



The only specimens of this shell that I have seen from the Ohio Coal 

 Measures are all crushed perfectly flat between the laminse of shale, so 

 as to obscure, to some extent, their characteristic features. The speci- 

 men figured seems to show the inside only of the right valve. It will 

 be seen to be larger than the Nebraska type specimens of the species, 

 and differs slightly in some of its details, but the latter differences are 

 believed to be mainly due to the condition of the specimen, and the fact 

 that it is probably the inner side that is seen. 



That these little American Coal-Measure shells are entirely distinct 

 from the European Permian species, A. pi7inxformis, will, I think, be 

 obvious enough without detailed comparisons. For remarks on this 

 point, however, I would refer the reader to the Nebraska Report cited at 

 the head of this description. Figures of the European form, for compar- 

 ison, are also given there. 



Locality and position : The original type specimens of this species was found in the 

 Upper Coal Measures, at Nebraska City, Nebraska. It also occurs at near the same 

 horizon in western Iowa, and at a somewhat lower horizon in the central region of 

 that State. The Ohio specimen here figured came from the lower part of the Coal 

 Measures. 



Genus PLEUROPHORUS, King, 1844. 



(Ann. Mag. Nat. Hist., XIV., 313.) 



Pleurophokus tropidophorus, Meek. 



Plate 19, figs. 10a, b. 



Shell transversely oblong, much compressed, with length a little greater 

 than twice the height; posterior margin flattened and bifurcated, the 

 lower truncation being nearly vertical, and the upper sloping obliquely 

 downward and backward from the hinder end of the hinge; cardinal 

 margin straight, equaling about two-thirds the length of the valves; 

 anterior rounded below and sloping abruptly forward from the beaks 



