No. 384.] CHELONIJAN CARAPACE AND PLASTRON. 939 
chelys, was less fruitful ; and I had about concluded that all 
vestiges of them had vanished. Finally, however, I was led to 
examine more closely the carapace of Macroclemys. These 
turtles alone among all living forms, so far as I know, possess 
a row of three or four epidermal scutes lying along each side 
between the costal scutes and the marginals. They are known 
as supramarginals. Each of these areas is lifted up into a 
rounded knob somewhat like the tubercles of the keels higher 
up. This row of knobs I regard as the last remaining vestiges 
in the Thecophora, of the second pair of keels of the ancestral 
turtle. This pair of keels in Dermochelys may properly be 
called the supramarginal keels. 
As to the third pair of lateral keels, they have probably left 
traces of themselves in the serrations that mark the margins of 
the carapace of many turtles, more especially the posterior 
margins. 
The ventral surface of Dermochelys is provided with five 
keels, two lateral on each side and a median. The two lateral 
pairs are most conspicuously developed in a young Dermochelys 
recently hatched. These keels, both dorsal and ventral, may 
be of some use in swimming, in maintaining the body in a direct 
course; but the adults of other sea turtles are without more 
than the merest traces of them. Nevertheless, some of the 
ventral keels are well developed in the young of the marine 
turtles. In a young Thalassochelys! the first pair of lateral 
keels runs along the middle of the hyo- and hypoplastral bones, 
and these keels are conspicuously tuberculated. In the same 
individual the keels of the second pair are seen to run along 
the rows of inframarginals and are also tuberculated. Rela- 
tively few turtles possess inframarginals, and it was the finding 
of the lateral keels in Thalassochelys that suggested to me an 
examination of the supramarginals of Macroclemys in my search 
for traces of the corresponding keels on the upper surface of 
the body. 
The plastron of Toxochelys possesses on each side a low but 
sharply defined keel, which corresponds to one of the first pair 
of Dermochelys. It is represented in Case’s figure. We should 
1 Agassiz, A. Cont. Nat. Hist., N. A., vol. ii, Pl. V, Figs. 14-16. 
