No. I.] RELATIONSHIPS OF PROTOSTEGA. 49 
from the stem of the Cheloniidae. Is it likely that the primi- 
tive form would have this process, lose it in the cretaceous, and 
regain it in the upper eocene and later forms? It is more 
likely that the Dermochelytdae arose after the process had 
appeared in the main stem, and the Pvotostegzdae before it. 
Nor is it likely that the process is secondary in either of the 
families possessing it. In the cases where such parts are 
seemingly reproduced in nature, it is generally found that some 
neighboring part has assumed the function and appearance of 
the lost part; there is nothing here that could have done this. 
Allopleuron connects Protostega with the main stem of the 
Chelonitdae ; it cannot be in the direct line, as it is upper cre- 
taceous, while Protostega is middle cretaceous; but its imme- 
diate ancestor must have been the connecting form. As shown 
in the description given above, and as will be more readily seen 
by reference to the plates in M. Ubagh’s two papers (41, 42), 
the skull is closely related to that of Thalassochelys, while the 
slender ribs and reduced pleurals are a direct step toward the 
state found in Pvotostega (see Pls. I, VII, XII, and XIII of 
Winkler’s monograph, 40). 
The point of approach of Protostega to the stem is indicated by 
the lacking process on thenuchal. In Osteopygis, the cretaceous 
form, the process is wanting, while in Lyto/oma of the upper 
cretaceous and in the form of the lower eocene it is present. 
Evidently Pvotostega branched off before the time of Lytoloma. 
Protosphargis of the upper cretaceous is the next step in the 
Protostegidae, as evidenced by its appearance in time, its simpler 
ribs, and reduced peripherals and plastron. 
Pseudosphargis is placed as the last of the known Pyvoto- 
stegidae because of the presence of the descending parietal 
plates and its appearance in the oligocene. The lack of any 
portion of the animal beyond the posterior part of the skull 
makes the determination of its systematic position doubtful. 
If it should prove to have a process on the nuchal plate for the 
eight cervical vertebrae and any trace of a dermal carapace, it 
might, though occurring so late, be regarded as representing 
the first step toward Eosphargis. This view would be supported 
by the weak attachment of the lower end of the parietal plates. 
