NATURAL SCIENCES OF PHILADELPHIA. 249 



in the absence of a mesial ridge extending from the beaks to the middle of its 

 basal margin, from that and other well determined species of the genus. 



Should it be found necessary to establish a new genus for this shell, we 

 would propose to call it Rhombocardia. We remember a similar, but distinct 

 species, from the New York Hamilton Group, which, if we mistake not, has been 

 described by Mr. Conrad, though we cannot just now recall its name. 



Locality and position. Bake-oven, Jackson Co., Illinois, Hamilton Group. 



Genus CONOCARDIUM, Bronn, 1837. 



CoNOCARDIUM OBLIQUUM, M. & W. 



Shell rather small, obliquely subtrigonal, gibbous ; anterior side (posterior 

 of Woodward) very obliquely and abruptly truncated with a forward slope, 

 and flattened so as to present a regular, cordate outline in a front view ; an- 

 terior auricle narrow, but of unknown length ; base very short ; posterior 

 margin sloping up from the base so as to intersect the hinge at an angle of 

 about 45, rather widely gaping, and crenate its entire length. Beaks mode- 

 rately prominent, small, strongly incurved ; umbonal slopes very prominent, 

 angular, and directed obliquely forward to the angular anterior basal extremity. 

 Surface ornamented with rather sharply elevated, threadlike, subcrenate radia- 

 ting ribs, narrower than the depressions between ; each of these depressions on 

 the posterior and flattened anterior sides of the valves occupied by a smaller 

 intermediate rib ; entire surface also marked by fine very regular radiating 

 and concentric striae, so as to produce a neat minutely cancellated sculpturing, 

 as seen under a magnifier. 



Length from the posterior extremity to the produced antero-basal angle, 

 0"70 inch ; height from the latter to the beaks, 50 inch ; length from the 

 beaks to the posterior extremity, 0-37 inch ; convexity, - 44 inch ; breadth of 

 posterior hiatus, - 17 inch. 



We know of no other species liable to be confounded with this. Its most 

 marked features are the great backward obliquity of its umbonal axis, by 

 which its beaks are placed even a little behind the middle of the body part of 

 the shell ; and the beautiful regular cancellated style of ornament seen be- 

 tween the ribs, under a magnifier. 



Locality and position. Coal Measures. Wabash Cutoff, Posey County, Ind. 



Genus EDMONDIA, De Koninck, 1842. 



EDMONDIA ? PEROBLONGA, M. &. W. 



Shell oblong, the length being about double the height, very inequilateral, 

 moderately convex ; the greatest convexity along the oblique umbonal slopes, 

 above and below which the valves are cuneate postero-dorsally, and antero- 

 ventrally. Posterior side distinctly compressed near the extremity, its mar- 

 gin rounded or subtruncate in outline ; anterior side very short, less com- 

 pressed and rather more narrowly rounded than the other ; basal and dorsal 

 margins nearly straight and parallel, the former being very slightly convex in 

 outline a little in advance of the middle. Beaks near the anterior end, very 

 oblique, compressed, and but slightly elevated above the hinge margin ; umbo- 

 nal slopes prominently rounded from the beaks obliquely to near the posterior 

 inferior margin. Surface of caA showing only faint traces of a few irregular 

 concentric undulations below the umbonal ridge. (Hinge and interior un- 

 known.) 



Length, 2-46 inch ; height 1-25 inch. Convexity of a left valve, - 47 inch. 



Although we have but a cast of this shell, showing neither the hinge, in- 

 ternal characters, nor the surface markings, we have thought it should be in- 

 dicated, as better specimens can scarcely be expected from such a matrix. 

 We confess, however, that we are totally at a loss respecting its generic charac- 

 ters, and merely place it provisionally in the genus EJmondia. 



1865.] 



