50 CASE. [Vou. XIV. 
In a slightly different direction developed those forms with 
the parietal plates absent, and dermal ossifications in the cara- 
pace and plastron culminating in Dermochelys. The first form 
to appear was Losphargis, slightly off the main line of the 
Dermochelys branch. The primitive form with parietal plates 
and lacking dermal ossifications has yet to be found to connect 
the branch with the main stem. From the strong resemblance 
of Eosphargis to Dermochelys it is fair to say that it had a 
process on the nuchal plate, and if so it must have come from 
a form which left the main stem after Lytoloma had developed 
the nuchal process. That it is slightly different from the rest 
of the branch is shown by the peculiar dermal carapace, con- 
sisting of the large carinated dermal scutes, instead of the 
small ones of Psephophorus and Dermochelys. It may be that 
these scutes are the last of the neurals; if so it represents a 
stage in its branch corresponding to a stage between AJ//o- 
pleuron and Protostega in the other branch. 
The second member of this branch is Psephophorus, running 
from the eocene to the pliocene; it has been shown by Dollo 
to be unquestionably a member of the Dermochelyidae, with 
well-developed dermal ossifications in the carapace, complete 
loss of neurals and peripherals, and a characteristic humerus. 
Dermochelys is the culminating form of this branch. The 
series Osteopygis, Lytoloma, Argillochelys, Thalassochelys, rep- 
resenting stages in the main line of the Chelonizdae, needs no 
further explanation than the facts set forth above. 
The ideas here expressed may best be seen by reference to 
the following diagram: 
Dermochelys Cheloniidae 
Psephophorus Thalassochelys Pseudosphargis 
Argillochelys 
Eosphargis 
ees Protosphargis 
Cisne Allopleuron is 
Lytoloma - 
i “ Protostega 
Osteopygis 
