Page:The Osteology of the Reptiles.pdf/116

98 its exogenous chevron is typically rhachitomous, in that it is wedge-shaped. And this very probably represents the real intermediate condition between the embolomerous and holospondylous vertebrae. Evidence that reptilian vertebrae arose in this way is also seen in the dorsal vertebrae of a young Seymouria, the most amphibian-like,



. 76. Vertebrae: A, B, C, Cricotus (Temnospondyli), dorsal, basal caudal, and median caudal, from the side and front. D, Eryops (Temnospondyli), caudal, from the side. E, Seymouria (Cotylosauria), median dorsal, from the side. F, Dimetrodon (Pelycosauria), dorsal intercentrum from behind and below. G, Trimerorhachis (Temnospondyli), intercentrum from side and below.

otherwise, of all known reptiles (Fig. 76 ). The intercentrum is here remarkably large for a reptile, nearly half as long as the notochordal centrum or pleurocentrum. And it is also almost the condition found in the first vertebra of primitive reptiles, the atlas (Fig. 79), as will be shown in the discussion of that bone. Additional evidence is furnished by the fact that while truly embolomerous vertebrae occur in fishes, in the modern Amia, for instance, real rhachitomous vertebrae