No. 384.] CHELONIAN CARAPACE AND PLASTRON. 933 
This view, however, is not without difficulties of its own. 
Dollo and Seeley have both referred to the fact that the dermal 
plastron of Dermochelys is not complete ; that is, the bony 
mosaic is deficient in the spaces between the longitudinal keels 
of the plastron. Shall we now regard this condition as a stage 
on the way toward a complete plastron ; or shall we hold that 
the complete plastron was once possessed and that the present 
condition was due to reduction? The former way of looking at 
the matter is opposed to the fact that Dollo and Seeley con- 
cluded that Psephophorus, a close ally of Dermochelys, living 
in the Miocene, possessed a continuous plastron of mosaic-like 
pieces. If the latter view is held, we might properly inquire 
how it happens that nature is so vacillating regarding the needs 
of this animal. 
We may well doubt, too, that there has been sufficient time 
granted the Dermochelyidz in which to effect the change in 
their armor. Case derives the family from Lytoloma, of the 
upper Cretaceous and lower Eocene ; but both Eosphargis and 
Psephophorus had appeared already in the Eocene. This im- 
plies rapid modifications of structure. On the other hand, it is 
evident that changes go on in the turtles very slowly. How 
much progress in the reduction of the carapace and plastron, 
for instance, has been effected in the Cheloniidz since Creta- 
ceous times? 
A difficulty affecting not only Baur’s later view, but also the 
earlier one, is experienced in endeavoring to understand what 
advantage Dermochelys has gained over the alleged old-fashioned 
turtles by undergoing its various supposed adaptive changes. 
The thecophore sea turtles are more numerous in genera, 
species, and individuals than the Athecz, notwithstanding the 
fact that the former have been relentlessly pursued for their 
flesh, their shell, and their eggs. As an ancient, intractable 
form, with difficulty adapting itself to its environment, we can 
understand Dermochelys. 
Seeley! has felt the necessity of accounting for the origin 
of the armor of turtles in a way different from that usually 
adopted. He thought that a portion of the carapace had its 
1 Quar. Jourp. Geol. Soc., vol. xxxvi, p. 410. London, 1880. 
