114 



THE OSTEOLOGY OF THE REPTILES 



Sauropterygia articulate with the diapophysis (d) exclusively by a 

 single head, the cervical ribs (Fig. 89 r) exclusively with the cen- 

 trum, usually also by a single head. 



The dorsal ribs of the Archosauria, that is, the Pseudosuchia, 

 Parasuchia (Fig. 88), Crocodilia (Fig. 90), Dinosauria (Fig. 80 p), 

 and Pterosauria, are double-headed, the anterior ones at least, but 

 both articulations are with the arch or diapophysis. And this mode 

 of articulation would seem to exclude their immediate ancestral re- 

 lationship to the birds, in which the head of the 

 ribs articulates with the centrum throughout. 



Atlantal ribs, present in all early reptiles, 

 have been lost in modern ones, except the Croc- 

 odilia, where they are attached exclusively to 

 the intercentrum, in the ancient Metriorhynchus 

 pz. to both arch and intercentrum. In the Dino- 

 sauria, some of them at least, the first inter- 

 centrum bears a small rib (Fig. 81). 



Axial ribs are more often present, but are lost 

 in not a few reptiles, particularly the Ptero- 

 sauria (Fig. 80 e) and Chelonia (Fig. 80 m). In 

 early crocodiles the axial rib articulated with 

 diapophysis and parapophysis ; in later croco- 

 diles the diapophysial articulation is lost, 

 though a vestige often remains, and the single- 

 headed rib has migrated forward on the odon- 

 toid. 



The dorsal ribs of the Eunotosauria and all 

 Chelonia ^ have expanded to meet or fuse with 

 each other, forming more or less of a carapace (Fig. 91). Peculiarly 

 expanded and overlapping ribs in the posterior dorsal series occur 

 in some of the Theriodontia. In Cynognathus the thirteenth to the 

 seventeenth ribs shorten rapidly and project widely with a remark- 

 able expansion near the proximal end, which overlaps the succeed- 

 ing rib in a concavity on its anterior border. In the lumbar series 

 (Fig. 92) they lose the free portion of the shaft, ending in wide, 



1 [This leaves out of account the costal plates which enter into the formation of the 

 carapace. See Gadow, " Reptiles and Amphibia," Cambridge Nat. Hist.; Procter, 

 1923, Proc. Zool. Soc. — Ed.] 



Fig. 88. Dorsal verte- 

 bra of phytosaur: az, 

 anterior zygapophysis; 

 pz, posterior zygapoph- 

 ysis; d, c, articulations 

 of rib. 



