280 PALEONTOLOGY OF OHIO. 



margins rounding into the front, and usually curving inward behind, so 

 as to intersect the hinge at rather more than right angles ; hinge gen- 

 erally a little less than the greatest breadth of the shell. Dorsal valve 

 always distinctly convex over all the central region, and thence round- 

 ing downward to the front and lateral margins, while its posterior lateral 

 edges are generally reflected or curved more or less upward ; beak round- 

 ing over to the hinge, beyond which it projects but little. Ventral valve 

 flattened, or more or less concave, with its lateral, and sometimes its 

 front margins, curved a little upward, and its posterior lateral downward, 

 to conform to the curvature of those of the opposite valve ; beak mod- 

 erately prominent, and directed obliquely backward and upward, but not 

 incurved ; cardinal area generally of medium height, flattened, well de- 

 fined, and inclined a little backward, with its closed triangular fissure 

 varying in its proportional breadth and height with the greater or less 

 elevation of the beak ; interior without any mesial septum ; muscular 

 impressions occupying a comparatively small, fan-shaped space, that is 

 neither deeply excavated nor bounded by prominent dental ridges. 

 Surface of both valves ornamented by numerous, sub-equal, or alternately 

 larger and smaller, radiating striae, that increase rather by the intercala- 

 tion of smaller ones between the larger than by division, the smaller 

 commencing very slender at various distances from the beaks and ex- 

 tending to the free margins, increasing in size, so that they often become 

 nearly or quite as large as the others. Crossing all of these are numer- 

 ous, very fine, crowded concentric striae, and obscure, much larger, ridges 

 of growth. 



Length of a rather large, wide, adult specimen, 1.73 inches ; breadth 

 of do., 2.16 inches; convexity of the two valves at the middle, 0.65 inch ; 

 do. of the dorsal valve, measuring from its lateral margins up to the 

 horizon of its most gibbous central region, 0.90 inch. 



the Imperial Society of Moscow, containing a description of a new genus which he 

 said was related to Placuna and Pedum, and proposed to call Orthotetes. He neither 

 figured, described, nor cited any example, however, and as it is not possible to know 

 from his description and remarks what genus he had in view, he can not be regarded 

 as having established a genus under that name at that time. It was simply a name 

 resting upon nothing. In 1837, however, Fischer uses the name Orthotetes, and fig- 

 ures a shell under it, without a specific name, but having almost certainly the char- 

 acters of Hemipronites. Again, in 1850, Fischer uses the name Orthotetes, and figures 

 an example with the name O. radians attached, which seems to be H. crenistria. But 

 as the geilus was thus only properly made known under the name Orthotetes in 1837 

 and 1850, while Pander had described it with illustrative examples in 1830, under the 

 name Hemipronites, I think his name should be retained, whether we regard the 

 group as a sub-genus or as a distinct genus. 



