932 THE AMERICAN NATURALIST. [Vou. XXXII. 
I shall not enter into any extended discussion of those struc- 
tures which Dermochelys possesses in common with the other 
sea turtles. Most of them have already received consideration 
from Baur, Dollo, Boulenger, Van Bemmelen, and Case. 
Some of these common characters may be attributed to inher- 
itance from a common remote ancestor; such are, for illus- 
tration, the wide separation of the pterygoids seen in both 
Protostega and Dermochelys, and the roofed-in condition of 
the temporal region found in Dermochelys and the Cheloniidz. 
Other characters possessed in common may be due to- conver- 
gence, resulting from similarity of habits, movements, etc. I 
would include in this category the presence of an articulation 
between the eighth cervical and the nuchal, the plane surfaces 
of articulation between the sixth and seventh cervicals, and 
the more or less reduced condition of the carapace and plas- 
tron. Probably the surfaces joining the sixth and seventh 
cervicals are plane for the same reason that they are plane be- 
tween the various dorsal vertebrz ; namely, this articulation is 
one situated where there is only slight movement, lying, as it 
does, between two curves in opposite directions. 
But whatever may be the conclusions reached concerning the 
other points in the anatomy of Dermochelys, its singular dorsal 
and ventral shields form one of the most striking characters of 
the animal, the one about which there has been the most con- 
tention, and the one which probably furnishes the key to the 
situation. The condition and mode of origin of this carapace 
were the most difficult matters for Baur to explain ; and regard- 
ing its morphogeny he changed his mind more than once. At 
the time the discussion was going on between himself and Dollo 
and Boulenger, Baur took the position that the carapace of 
Dermochelys had been derived from that of its Chelonioid 
ancestors through delamination of the layer of membrane bone 
from the ribs, and the dissolution of this into polygonal pieces. 
Later he came to the conclusion that the membrane bone of the 
carapace of Dermochelys had become wholly, or nearly wholly, 
reduced, and that the layer of mosaic-like pieces was of second- 
ary origin, an entirely new development. Case adopts this 
later expressed opinion. 
