334 PALEONTOLOGY OF OHIO. 



shells often vary much in form, I prefer not to attempt to separate them 

 specifically without knowing from the study of a series of specimens the 

 extent to which this species varies. To prevent confusion, however, in 

 case they may be found to belong to two distinct species, I would state 

 that the form represented by fig. 7a is considered the type of the species 

 here described. 



Locality and position : In dark shales of the Coal Measures at Flint Eidge, Ohio. I 

 think I have also seen imperfect specimens of the same shell from the Coal Measures 

 of Illinois. 



Genus MAGTiODOl^, Lycett, 1845. 



[Buckman in Murch. Geol. Chelt., 2d ed.] 



Mackodon obsoletus, Meek. 



Plate 19, fig. 9. 



Macrodon obsoletus, Meek (1871) ; List Carb. Fossils from West Virgi^iia, 5 (Extr. fi*ora 

 Rep. Regents University of West Virginia). 



Shell (as determined from internal casts) transversely elongated sub- 

 rhombic, rather compressed; hinge line nearly equaling the length of 

 the valves; posterior margin compressed, obliquely truncated, and very 

 slightly, so as to meet the hinge above at nearly a right angle, and 

 subangular or very abruptly r.;unding into the base below; anterior 

 margin intersecting the hinge ai a right angle above, thence rounding 

 downward into the base, which is not quite parallel to the dorsal margin 

 and slightly sinuous near or a very little in advance of the middle ; beaks 

 depressed very nearly to the line of the dorsal margin, rather compressed, 

 and placed about one-fourth the length of the hinge from the anterior 

 extremity; surface showing a few distant concentric marks, traces of fine 

 striae of growth, which are crossed on the posterior dorsal region by fine, 

 rather closely arranged, radiating striae. 



Length, 1.40 inches; height, 0.58 inch; convexity, about 0.22 inch. 



The original type specimen of this species is only about two-thirds as 

 large as that here figured, which latter is only a partly internal cast of 

 a right valve. This cast shows the impressions of two or three elon- 

 gated posterior hinge teeth, nearly parallel to the cardinal margin. In 

 a cross light it also shows obscurely the fine, radiating and concentric 

 striae of the posterior dorsal region, the former of which escaped the eye 

 of the artist when drawing the figure. On this part of the type speci- 

 men these striae form a finely cancellated style of ornamentation. 



