670 GEOLOGY OF OHIO. 



inner side by a ridge that is not known in Cyrtodanta. Nor has that 

 genus a small pedal muscle over the anterior adductors. Finally the 

 two adductors are more nearly equal in size. Practically the same differ- 

 ences, besides others that it is not necessary to point out, obtain when 

 compared with Vanuxemia, Billings and Cypricardites, Conrad. 



It seems to me that Ortonella is nearest Ischyrodonta, Ulrich. Only 

 two differences have I been able to find. Namely, the escutcheon and 

 lunule, and the posterior lateral teeth, both of which are wanting, in 

 Ischyrodonta. In all other respects however the shells in the two genera 

 seem to be identical even to the smallest detail. 



OrloneLa hainesi, S. A. Miller. 



Plate 53. Figs. 9-18. • 



Cypricardites hainesi, S. A. Miller, 1874, Cin. Quart. Jour. Sci., vol. 1, p. 147. 



This interesting species is so fully illustrated on plate 53, that a de- 

 scription is unnecessary. It will be well however, to compare it some- 

 what carefully with species of Ischyrodonta that are to be found at the 

 same locality, though not in exactly the same strata. 



I refer especially to Ischyrodonta decipiens^n. sp., and /. truncata, 

 Ulrich. Both may be distinguished at once, providing the specimens 

 are complete enough, by the fact that their hinges are without posterior 

 lateral teeth, Ortonella having two strong laterals as shown in figures 11 

 and 12. Even with respect to this point, the student is cautioned against 

 the possibility of mistaking the linear ridges which served as supports 

 for an internal ligament in Ischyrodonta (see plate 54, figures 16 and 19) 

 for lateral teeth. That they could not have served as hinge teeth is 

 proved by the fact that they do interlock. Indeed, the corresponding 

 ridges in the two valves are separated by an interval, the valves being in 

 contact dorsally only at the outer margin. Externally the Ortonella is 

 recognized principally by the lunule and escutcheon, but it differs also 

 in its stronger umbonal ridge and in the outline, the basal line being 

 somewhat straighter and the anterior end higher so that the dorsal and 

 ventral margins are more nearly parallel. There is besides a difference 

 in the surface markings, the concentric lines in the Ischyrodonta being 

 much coarser. 



Formation and Locality. — Ortonella hainesi is associated with Rhyn- 

 chonella dentata Hall, at Richmond, Indiana, where both species, though 

 common fossils, are restricted to a few feet of strata, less than forty feet 

 beneath the extreme top of the Cincinnati group. Ischyrodonta truncata 

 seems likewise to be restricted to this bed, but the /. decipiens, which is 

 most like the O. hainesi has so far been found only in beds at least 

 twenty feet above the Ortotiella horizon. 



