I04 



THE OSTEOLOGY OF THE REPTILES 



centrum are fused into a ring, which revolves about its pleurocen- 

 trum, 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. In no reptile did the atlas attain the spe- 

 cialization 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 ro- 

 tation about it, not more than 

 between the axis and the fol- 

 lowing vertebra. This lack of 

 torsion, in most reptiles at 

 least, was compensated for 

 by the ball-and-socket joint 

 between the single condyle 

 cand the atlas, lost in mam- 

 mals. 



In the primitive Ophiaco- 

 don (Fig. 78) and Dimetrodon 

 (Fig. 79) the condylar cup is 

 formed by the intercentrum 

 and arch, completed in the 



Fig. 81. Atlas and axis of D//)/o^o<:;/j(Saurischia). middle by the frOUt end of 

 After Holland. One fourth natural size. ^^^ odoutoid, 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 noto- 

 chord, as in the embryonic cartilage of mammals. The pleurocen- 

 trum 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 Rhyn- 

 chocephalia (Fig. 80 d), Choristodera (Fig. 80 f), and Phytosauria 

 it is yet largely visible from the side, but the first and second inter- 

 centra have become contiguous below it. In the Crocodilia (Fig. 

 80 g) and Chelonia (Fig. 80 m) the pleurocentrum still retains its 



