148 EXTINCT VERTEBRATA FROM THE JUDITH RIVER 
The fossil tooth (figs. 53-55, plate 9,) bears much resemblance to one of the lateral 
denticles of the teeth of the great extinct shark, Carcharodon angustidens, and under 
other circumstances might readily have been mistaken for such. 
The specimen is black and shining, and is laterally compressed, conical, and curved 
backwards, as observed in the Monitor ornatus. The margins cf the tooth are trenchant, 
and strongly denticulated; the denticles possessing the same form as the crown itself. 
On the convex border of the tooth there are eleven denticles, and on the concave border, 
seven; and on both borders the points of the denticles diverge upwardly. 
The broken base of the crown is elliptically trapezoidal, and is hollowed on the interior. 
The crown is invested with enamel, which on one side of its summit is worn off by the 
attrition of an opposing tooth passing it like the blades of a pair of scissors. The length 
of the specimen is 3 lines; its antero-posterior diameter at base, 2 lines; and its trans- 
verse diameter, 12 lines. 
I have no evidence that part of or all of the vertebrae supposed to belong to Palaoscin- 
cus, do not really appertain to 7’roodon. This question must be left for future investiga- 
tion to determine. 
Explanation of Figures, Plate 9. 
Figures 53—55. Tooth of TRoopon rorMosus; magnified three diameters. 
Figure 58. Outer view. 
Figure 54. Inner view, exhibiting the enamel worn from the summit. 
Figure 55. Section at the base of the specimen. 
CHELONIA. 
TRIONYX FOVEATUS. 
Among the fossils of Dr. Hayden’s Judith River Collection, there are a number of small 
fragments of costal and sternal plates, having much resemblance to the corresponding parts 
of our living soft-shelled Turtles, forming the genus 7rionyx. 
The exterior surface ef the fragments of costal plates, (figure 2, plate 11,) is impressed 
with shallow pits, except near the borders of the plates. The pits are smaller and rounded 
at the vertebral extremities of the latter, and become larger outwardly, assuming a poly- 
hedral, often oblong and reniform outline. The fragments of the sternal plates, (figure 
1, plate 11,) have their exterior surface covered with short vermicular ridges, which re- 
call a remote appearance to Arabic letters. One of the fragments of a costal plate, 
apparently the third or fourth, represented in figure 2, is almost 11 limes wide, and 2 
lines thick. Two fragments of a hyposternal plate, (figure 1,) are 3 lines in thickness. 
