centrum are fused into a ring, which revolves about its pleurocentrum, the odontoid, a small, tooth-shaped, or spout-shaped bone firmly fused with the axis in front and usually described as a part of it. Long ago, however, the odontoid was recognized by Cuvier as really the body of the axis.
Fig. 81. Atlas and axis of Diplodocus (Saurischia). After Holland. One fourth natural size.
In no reptile did the atlas attain the specialization of the mammals, even approximately, but it most nearly approached it in the Theriodonts. In very few do the two bones of the arch fuse with the intercentrum into a complete arch ring, or does the pleurocentrum unite with the axis as a real odontoid. In few is there any degree of rotation about it, not more than between the axis and the following vertebra. This lack of torsion, in most reptiles at least, was compensated for by the ball-and-socket joint between the single condyle and the atlas, lost in mammals.
In the primitive Ophiacodon (Fig. 78) and Dimetrodon (Fig. 79) the condylar cup is formed by the intercentrum and arch, completed in the middle by the front end of the odontoid, that is, the pleurocentrum or true centrum, which has no independent motion whatever, and is not united with the axis. The arch bears a rib upon its diapophysis, and the large odontoid is perforated for the notochord, as in the embryonic cartilage of mammals. The pleurocentrum or centrum, large and notochordal primitively, reaching the ventral side of the vertebra, grew progressively smaller till it finally disappeared wholly from side view in the Pterosauria (Fig. 80 e), most Dinosauria, and the Squamata (Figs. 80 b, l). In the Rhynchocephalia (Fig. 80 d), Choristodera (Fig. 80 f), and Phytosauria it is yet largely visible from the side, but the first and second intercentra have become contiguous below it. In the Crocodilia (Fig. 80 g) and Chelonia (Fig. 80 m) the pleurocentrum still retains its
