. Carnegie Institution of Washington publication. . Fig. 246.—Protostcva patens. Entoplastron and xiphiplastron of type. X i. ent, entoplastron; xiph, xiphiplastron. 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 th
. Carnegie Institution of Washington publication. . Fig. 246.—Protostcva patens. Entoplastron and xiphiplastron of type. X i. ent, entoplastron; xiph, xiphiplastron. 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: (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 of three of these cases it was in its apparently natural position with relation to the hyoplastra. ince this sent to the puss Dr. Wieland has publisht a paper (Ann. Carnegie Mus , iv, 1006, p. 8) in which he accepts
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