The Skull of the Chelonia
(Figs. 30–32)
Fig. 32. Trionychoid skull: Platypeltis. Natural size. A, from above;
B, from below; C, atlas of same from the side. Note abnormal proatlas.
The skull of the Testudinata or Chelonia is never elongate, though relatively slender in some of the more predaceous kinds. It always lacks the septomaxillae, postfrontals, supratemporals, dermosupraoccipitals, tabulars, ectopterygoids, ethmoids, and the parietal foramen. The nasals are usually absent, and the lacrimals are present only in some ancient forms. The prefrontals are large, meeting in the middle line. The prevomers are single and sometimes obsolete; they usually articulate above with the prefrontals. The parasphenoid has been lately recognized as a distinct bone in certain forms. There is no postoptic, but its place is taken, except in Dermochelys, by descending plates from the parietals to the pterygoids, sometimes with an intercalated epipterygoid, which, however, is usually absent. The quadrate may or may not reach the basisphenoid. The palatines often meet in the middle line between the pre-
