DESMATOCHELYID&. 18g 
hardly separated from the head. The ulnar process is nearly parallel with the long axis of the 
bone. The head is farther removed from the axis than in Ne eptunochelys. In these two 
respects Atlantochelys resembles Desmatochelys; but it differs from the latter in having a 
slender shaft. This humerus must have been fully as long as that of Archelon, but it was of 
very different form. A very large turtle is indicated. 
Family PROTOSTEGIDZ Cope. 
Marine turtles with the fore limbs converted into flippers resembling those of the Che- 
loniidz. Carapace greatly reduced, the disk extending not one-half the distance toward the 
distal ends of the ribs. Peripherals present. Plastron loosely connected with the carapace 
and with a large median fontanel. Entoplastron T-shaped, with the lateral wings elongated 
and distally expanded. Epiplastra not certainly known. Xiphiplastra short and bent. Skull 
large, temporal region broadly rooft over. Region i in front of the orbits elongated. Jaws with 
large crushing-surfaces. Choanz far fovwacd: not underfloored by the surrounding bones. 
Genera: Protostega, Archelon, and probably Protosphargis, all of the Upper Citaccous: 
The writer is strongly of the opinion that the genera Protoste gaand Archelon ought to be 
kept apart from the syodein sea-turtles as a dance family. Vat both the Protostegide 
ee 
Fic. 246.—Protostega potens. Entoplastron and xiphiplastron of type.» 5. 
ent, entoplastron; xiph, xiphiplastron. 
and the Cheloniide had their origin from a common ancestor, which was a sea-going turtle, 
need not be denied; but the two branches have been separated so long, and each has developt 
so many peculiarities, that it seems unwise to force them into the same family. That the 
Cheloniide have been derived directly from the Protostegida it 1s impossible to believe. The 
latter family was in several respects more highly difereneated than the living sea-turtles. 
The greatly reduced carapace, the peculiar entoplastron, the abbreviated xiphiplastra, the 
large preorbital region, and the strongly modified humeri are examples of these differentiations. 
For the discussion of the relationships of this family to the Chelonidz and to the Der- 
mochelyidz, the reader is referred to the papers of Baur, Case, Furbringer, Van Bemmelen, 
Boulenger, Lydekker, Dollo, Wieland, and Hay. 
The bone which is here regarded as the entoplastron (fig. 246, ent) was, in Protostega, 
originally described by the present writer, on the advice of Dr. Baur, as the nuchal. ‘This 
view was adopted by Case and afterwards by Wieland.* The latter, in describing Archelon, 
originally held the bone to be the coalesct epiplastra and entoplastron, called by him the 
paraplastron. A re-examination of the subject, in the light of all the known materials, has 
led the present writer to change his opinion. The reasons for regarding this bone as the 
entoplastron are the following: 
(1) The bone has never been found in direct connection with peripheral bones. 
(2) It has been found four times closely associated with plastral bones, and in two o1 
three of these cases it was in its apparently ‘natural position with relation to the hyop ylastra. 
5f 
*Since this was sent to the press Dr. Wieland has publisht a paper (Ann. Carnegie Mus., iv, 1906, 
p. 8) in which he accepts the view that the bone is the entoplastron. 
