,054 GEOLOGY OF OHIO. 



.anteriorly, situated well forward, on a line with the back, scarcely incurved, 

 separated by a well defined, wide, channel-like depression extending pos- 

 teriorly from the points of the beaks half the length of the cast. Umbonal 

 ridge and sulcus strong, extending from the beaks obliquely downward 

 -to the central third of the base; and producing a decided compression of 

 the antero-basal third of the cast. Anterior muscular scar very strong, 

 obliquely ovate, large, occupying the greater part of the small anterior 

 .end. The inner side of the elevated scar is marked with about six hori- 

 zontal folds. Pallial line and posterior adductor impression indistinct. 

 The casts exhibit no indications of the surface markings. The hinge 

 plate seems to have been strong, while of cardinal teeth the evidence at 

 "hand indicates two in each valve, one larger than the other. 



The casts of this species might be confounded with those of small 

 specimens of Ischyrodonta elongata, Ulrich, but a careful comparison will 

 show that the Modiolodon is narrower posteriorly, the dorsal and ventral 

 margins being more nearly parallel and also straighter; the ventral mar- 

 gin is not sinuate, and there is no small pedal muscular scar above the 

 anterior adductor impression, while the inner side of the latter is thrown 

 into folds instead of being sharply edged. Excepting the following 

 -species there is no other known to me with which it need be compared. 



That M. subrectus is not an Ischyrodonta is shown by the black film 

 -so characteristic of the Modiolopsidce which is retained by two of the 

 -specimens. The absence of the small pedal muscles over the anterior 

 adductor impressions also is significant. 



Formation and locality: Upper beds of the Cincinnati group, Rich- 

 mond, Indiana. 



Modiolodon declivis, n. sp. 



Plate 53. Figs. 3 and 4. 



Of this species also only casts of the interior have been seen. These 

 are so much like those of M. subrectus that a detailed description is un- 

 necessary. On comparing the casts we find that M. declivis is more elon- 

 gate, the length being twice as great as the height; the ventral margin is 

 slightly sinuate instead of straight, and the dorsal margin arcuate, the 

 posterior part sloping downward in a manner quite unusual in this family 

 -of shells. The two ends are nearly equal, the posterior one being there- 

 fore relatively narrower than in M. subrectus. 



Formation and locality: Upper beds of the Cincinnati group, Rich- 

 mond, Indiana, where five specimens were collected. 



Modiolodon obtusus, n. sp. 



Plate 52, Figs. 20 and 21. 



Modiolopsis modiolaris, Hall and Whitfield, 1875, Pal. Ohio, Vol. II, plate II, Fig. 17. 

 (Not M. modiolaris, Hall, 1847, nor Pterinea modiolaris, Conrad, 1838.) 



Shell large, compressed-convex, oblong, subovate or obscurely quad- 

 rangular, highest behind, though unusually wide and blunt in front. Car- 



