674 GEOLOGY OF OHIO. 



lunule and escutcheon while the margin of the valves of the Ischyrodonta 

 are not in the least inflected; finally, that shell has a different hinge, 

 having true posterior lateral teeth. 



Compared with species of this genus, /. ovalis will be found to have 

 a thinner hinge plate and more regularly oval shape, while /. truncaia is 

 a higher shell, with fewer concentric surface markings, and much more 

 oblique cardinal teeth. 



Formation and Locality. — Near the top of the Cincinnati group, 

 Richmond, Indiana. 



Ischyrodonta ovalis, Ulrich. 



Plate 54. Figs 12-15. 



Ischyrodonta ovalis, Ulrich, 1S92, Nineteenth Ann. Rep. Geol. and Nat. Hist. Snr. 

 Minn., p. 242. 



Shell small, moderately veutricose, almost regularly elliptical, with 

 the greatest heighth and thickness subcentral; heighth and length about 

 as two is to three. Occasionally the ventral margin is less convex than 

 is the case in the specimen figured on plate 54. Beaks small, situated 

 near the anterior extremity, compressed by a flattening of the surface 

 which, expanding, extends over the greater part of the ventral slope. 

 Umbonal ridge strongly rounded, not however prominent enough to con- 

 stitute a conspicuous feature; cardinal slope abrupt, very little concave. 

 Surface marked with strong lines of growth and a few finer concentric 

 striae, both somewhat irregular. Hinge plate not very strong, arcuate, 

 widening posterior to the beaks and grooved for the reception of the in- 

 ternal ligament. Cardinal teeth two in the right valve, projecting down- 

 ward and backward from the hinge plate, which is thin at this point, 

 and supported by an internal process that seems to extend up into the 

 rostral cavity and projects on each side of the teeth so as to give the 

 whole the appearance of a quadrifid tooth. Anterior muscular scar 

 rather small, occupying the anterior end of the valve. Posterior scar 

 and pallial line not observed. 



In the original description of this species I stated that it was not 

 strictly congeneric with the types of Ischyrodonta. Additional speci- 

 mens and further comparisons however have convinced me of the error 

 of that statement, and I now regard the species as an unequivocal 

 member of the genus. Specifically it differs from all the other species 

 now referred to the genus, excepting /. itnionoides, Meek, sp., by its more 

 regularly oval shape. The excepted species is too different in other re- 

 spects to require comparisons. The associated /. decipiens is, I believe, a 

 much nearer species, but, as may be seen by comparing the figures on 

 plate 54, their outlines are quite different in the post-cardinal region and 

 the hinge plate of /. oralis comparatively weaker. 



For/nation and Locality. — Uppermost beds of the Cincinnati group, 

 near Richmond, Indiana. 



