390 C. R. Eastman — On the Genus Peripristis. 



1870. Feripristis semicircularis, 0. H. St. John: Proc. Amer. Phil. Soc, vol. xi, 



p. 434. 

 1872. Peripristis semicircularis, 0. H. St. John: Hayden's Final Eept. U.S. 



Geol. Surv. Nebraska, p. 242, pi. iii, figs. 3, 4 ; pi. iv, fig. 20. 

 1875. Ctenoptychius semicircularis, J. S. Newberry: Eept. Geol. Surv. Ohio, 



vol. ii, p. 52, pi. Iviii, fig. 14. 



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

 snpposed upper and lower teeth of this species are identical with those 

 known to obtain in P. falcatus, a specimen of the latter having been 

 described by Davis which displays the 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, the same 

 condition being true of Janassa, and of sharks generally. The 

 lower tooth of P. semicircularis (Fig. 1) differs from the upper in 

 having the serrations of the cutting edge obsolete, or nearly so, 

 and the inferior coronal margin deflected downward in the median 

 line in front. Its root, also, is longer than that of the upper tooth. 

 The trenchant margin of the latter is always strongly serrated in 

 unworn specimens, there being usually four denticulations on one 

 side of the median line and five on the other ; the largest of these 

 may be either centi-al, or in some cases slightly eccentric in position. 

 The coronal cavity of the upper tooth exhibits a deep pit in the 



Fig. 1. 



i 



Fig. 2. 



Fig. 1. — Feripristis semicirctilaris (N. & W.). Chester Group : Caldwell County, 

 Kentucky. «, lateral aspect of lower tooth; b, anterior view: nat. 

 size. (The indentations of the coronal margin are caused by contact 

 with the serrated edge of the upper tooth, which closed outside the lower.) 



Fig. 2. — Peripristis benniei (Ether.). Upper Carboniferous Limestone: Eichmond, 

 Yorkshire. Posterior aspect of upper tooth, nat. size. 



median line at the junction of the horizontal and vertical portions 

 of tlie posterior face, but there is no groove extending from it on 

 either side as in P. falcatns. Occasionally this pit is developed into 

 a perforation passing entirely through the horizontal portion of the 

 crown. The close contact between upper and lower teeth is indicated 



