DESMATOCHELYID.*. 



189 



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 Neptunochelys. 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 PROTOSTEGID^ Cope. 



Marine turtles with the fore limbs converted into flippers resembling those of the Che- 

 loniidae. 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 in front of the orbits elongated. Jaws with 

 large crushing-surfaces. Choanae far forward; not underfloored by the surrounding bones. 



Genera: Protostega, Archelon, and probably Protosphargis, all of the Upper Cretaceous. 



The writer is strongly of the opinion that the genera Protostega and Archelon ought to be 

 kept apart from the modern sea-turtles as a distinct family. That both the Protostegidae 



FlG. 246. Protostega potens. Entoplastron and xiphiplastron of type. X i- 

 ent, entoplastron; xiph, n'phiplastron. 



and the Cheloniidae 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 

 Cheloniidae have been derived directly from the Protostegidae it is impossible to believe. The 

 latter family was in several respects more highly differentiated 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 Cheloniidae and to the Der- 

 mochelyidae, the reader is referred to the papers of Baur, Case, Fiirbringer, 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: 



(i) The bone has never been found in direct connection with peripheral bones. 



(a) It has been found four times closely associated with plastral bones, and in two or 

 three of these cases it was in its apparently natural position with relation to the hyoplastra. 



*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. 



