~ 
935 THE AMERICAN NATURALIST. (NOL XXXII. 
If we examine the carapace of a specimen of Dermochelys, 
or, in lieu thereof, the plates presented by Gervais,! we shall. 
find that along the mid-line of the dorsal surface there is a row 
of enlarged bony plates, each bearing on its upper surface a 
prominent ridge. If we suppose that the early ancestors of our 
common turtles had at once a carapace like that of Dermochelys 
and a more or less rudimentary carapace of the common kind, 
it will be easy to comprehend that, as the dermal carapace 
underwent reduction, some of the larger median plates remained 
behind, first as independent ossicles, then as mere knobs on 
the neuralia. 
A further examination of the test of Dermochelys proves 
that on the upper surface there are six more keels, three on 
each side. The third keel on each side, reckoning from the 
mid-line, forms the margin of the carapace ; the two others are 
between it and the median keel. Each of these ridges, or 
keels, is composed of polygonal bony plates considerably larger 
than those occupying the spaces between the keels. 
In a considerable number of species of turtles there are to 
be found on the carapace three keels, a median and two lateral. 
The lateral keels, although usually not so prominent as the 
median keel, are sometimes quite as well developed, and are 
occasionally conspicuously tuberculated. The median and the 
lateral keels are to be seen in young individuals of the snapper 
(Chelydra), and are strikingly displayed in large specimens of 
the alligator snapper (Macroclemys). In the latter species the 
tubercles are very large and projecting. Other tricarinate 
species are Staurotypus triporcatus, Damonia reevesii? and 
Nicoria trijuga.4 These lateral keels occupy exactly the posi- 
tion held by the first pair of lateral keels of the carapace of 
Dermochelys ; and it seems to me entirely probable that they 
have been inherited from a common ancestor, and have been 
produced from rows of distinct ossicles, as the middle keel has. 
A search among various genera of thecophore turtles for 
traces of the second pair of lateral keels, as seen in Dermo- 
1 Nouv. Archives du Museum, vol. viii (1872), Pl. IX. 
2Gray. Catalogue Shield Reptiles, pt. i, Pl. XX B 
8 OP. cit., Pl. V. t Op. cit., Pl. IV. 
