COAL-MEASURE SPECIES. 333 



in the same genus. Baron de Reyckholt probably saw in some of his 

 specimens characters distinguishing his genus from Crania, but if I were 

 to form an opinion from his figures only, I should almost be led to think 

 they represented a thin species of that genus. 



Locality fwd position: Coal Measures at Flint Ridge and 'Putnam Hill, Ohio. 

 Prof. Andrews's collection. 



Genus POSIDONOMYA, Brown, 1837. 



(Leth. Geogn., 8S, 164 and .342.) 



POSIDONOMYA FRACTA, Meek. 



Plate 19, figs. 7a, b. 



Shell obliquely subovate, compressed, very thin; posterior basal mar- 

 gin regularly rounded J posterior dorsal edge ascending obliquely forward 

 to the hinder extremity of the hinge, which it meets at an obtuse angle ; 

 anterior margin descending or truncated more or less nearly vertically 

 from the beaks above, and rounding obliquely into the base below; hinge 

 line straight, very short, and ranging at an angle of about 45° to 60° 

 above the longer oblique axis of the valves ; beaks terminal, very oblicjue, 

 and projecting very little or not at all above the hinge margin; surface 

 marked by regular concentric undulations, with intermediate parallel 

 strise. 



Length of a narrow right valve, 0.72 inch; breadth of same, 0.43 inch ; 

 length of hinge, 0.22 inch. 



As may be seen by the figures, this little shell presents exactly all the 

 external characters of an oblique Inoceramus, the smaller right valve rep- 

 resented by fig. 7a being, in form and ornamentation, an exact miniature 

 of the well known cretaceous Inoceramus prohlematicus. I know nothing 

 of the hinge of these shells, but there is, of course, no probability that 

 they belong to the genus Inoceramus, and it is scarcely possible that 

 I. problematicus, which is unknown in any intermediate position, could 

 occur both in the Cretaceous and Carboniferous rocks, even if the genus 

 had so great a range in time, of which we have no evidence. I therefore 

 refer this shell to the genus Fosidonomya, to which it much more prob- 

 ably belongs, though its generic relations cannot be considered definitely 

 settled until its hinge characters can be determined. 



It will be observed that our figures represent a right and a left valve, 

 that differ quite enough in form to belong to different species, but as 

 they are opposite valves (of course not of the same individual), and such 



