940 THE AMERICAN NATURALIST.  [Vou. XXXII. 
hardly expect a huge sea turtle like Protostega to possess a pair 
of plastral keels ; but that such were present may be seen from 
the examination of my figures of this plastron.! A few turtles 
which are fitted for existence on the land also have these keels. 
They may be seen in Gray’s figures of Kachuga lineata and 
K. dhongoka* I find no tubercles that furnish evidences of 
remains of the median ventral keel in any turtles except Dermo- 
chelys. This keel appears to have quite completely vanished. 
I shall, however, return to a consideration of it. All the keels, 
as we now find them in the Thecophora, I look upon as having 
originated through the fusion of rows of distinct dermal ossicles 
with the underlying bones of the carapace and plastron. 
The presence, in turtles of so many and so widely removed 
families, of these keels and rudiments thereof, always more or 
less tuberculated at an early stage of life, is rendered compre- 
hensible if we once admit that the common ancestors of the 
groups possessed corresponding rows of tuberculated bones. 
On the other hand, the possession of these numerous keels by 
Dermochelys is, we might say, incomprehensible if we are to 
suppose that it took its origin from a race of sea turtles that 
had completely, or nearly completely, lost the carapace and plas- 
tron. If the structure of the new carapace had anything to 
do with that of the old one, and if the keels of the superior 
second lateral pair were really associated with the supramarginal 
scutes, how could these keels have reappeared in Dermochelys 
if this were derived from a stock which had no supramarginals 
or supramarginal keels? If the disposition of the new carapace 
had nothing to do with that of the old, how came it that we can 
seem to find such close correspondences? Dermochelys would 
offer a most remarkable case of convergence or reversion. 
One of the most remarkable facts about turtles is the want 
of correspondence between the horny scutes of the carapace 
and plastron and the bones which underlie them. When osteo- 
dermal plates are developed in the crocodiles and lizards, they 
are overlain by corresponding horny scutes. In tortoises, on 
the contrary, each lateral horny scute of the carapace covers 4 
1 Field Columbian Museum Pubs., Zool., vol.i, Pls. IV, V. 
2 Catalogue Shield Reptiles, pt. i, Pls. XVII, XVIII. 
