EASTMAN : CARBONIFEROUS FISHES FROM THE CENTRAL WEST. 179 



1872. Peripristis xemicircitlaris 0. H. St. John, Hayden's Final Kept. U. S. Gcol. 



Surv. Nebraska, p. 242, PI. III., Figs. 3, 4, PI. IV., Fig. 20. 

 1875. Ctenoj)ti/ckius semicirculnris J. S. Newberry, Hept. Geol. Surv. Ohio, Vol. II., 



p. 52, PI. LVIIL. Fig. 14. 

 1902. Periprislis semicircularis C. R. Eastman, Geol. Mag. (IV.), Vol. IX., p. 389, 



Text-fig. 1. 



It is evident from marks of contact that the relations between the supposed 

 upper and lower teeth of this species are identical with those known to obtain 

 in P.falcatvs, a specimen of the latter having been found which dispU\ys the 



Fig. 7. 



Peripristis semicircularis (N. & W.). Chester Group, Kentucky. Lower tootli, in 



profile and front view, X I- 



dental plates of both jaws in natural association. The tooth which may be 

 provisionally referred to the lower jaw in all these forms is the one which 

 fitted inside that of the opposite jaw when the mouth was closed, this condition 

 having been ascertained to hold in the case of Janassa, and being true among 

 sharks generally. The lower tooth of P. semicircularis differs from the upper 



Fig. 8. 



Peripristis semicircularis (N. & W.). Chester Group, Kentucky. Upper tooth, in 



profile and front view, X x- 



in having the serrations of the cutting-edge obsolete, or nearly so, and the 

 basal border deflected downward in the median line in front, as shown in Text- 

 figure 7. It also has a longer root than the upper tooth. The coronal margin 

 of the latter is always strongly serrated in the unworn condition (Text-fig. 8), 

 there being usually four denticulations on one side of the median line and five 

 on the other. The coronal* cavity of the upper tooth exhibits a deep pit in 

 the median line at the junction of tiie horizontal and vertical portions of the 

 posterior face, but there is no groove extending from it on either side as in 

 P. falcatus. In one specimen, that shown in Plate 2, Fig. 7, the pit is de- 



