THE SKELETON OF REPTILES 



33 



The ribs of reptiles are of more importance in classification than 

 one would suppose. The primitive rib was a slender, curved bone, 

 with the vertebral end dilated to articulate continuously with the 

 intercentral space — that between the centra and the anterior 

 part of the arch. And this is the condition still remaining in the 

 tuatera. Very soon, however, the lower end of the articular 

 surface (capitulum) became separated from the upper (tubercle) 

 by a notch, and the ribs became distinctly double-headed. And 

 this mode of articulation is the rule among mammals. Among 

 later reptiles, however, there were many modifications. In nearly 

 all the head migrated a little backward on the centrum. By the 

 loss of the tubercle in lizards, the head became truly single-headed, 

 and attached solely to the body ; 

 and this condition is character- 

 istic of the order Squamata. In 

 another large group the head of 

 the rib gradually migrated up on 

 the arch and on the transverse 

 process (diapophysis), so that 

 both head and tubercle are 

 attached to the diapophysis; 

 and this condition is equally 

 characteristic of the orders of 

 reptiles known collectively as 

 the Archosauria — the crocodiles, 



pterodactyls, dinosaurs, and phytosaurs. In the Sauropterygia, the 

 ribs are single-headed and attached to the end of the diapophy- 

 sis. Finally in most ichthyosaurs the capitulum and tubercle 

 both articulate with the body of the vertebra. 



Ribs primitively were probably attached to all the vertebrae 

 to the end of the tail. In the earliest reptiles that we know they 

 are present on all vertebrae as far back as the tenth or twelfth 

 caudal only, those of the caudal for the most part co-ossified 

 with the centra. The ribs of the neck vertebrae more quickly 

 disappeared, or became fused with the vertebrae, and only in the 

 crocodiles among living reptiles are there ribs on the atlas. The 

 sacral ribs, on the other hand, became much larger and stouter and 



Fig. 17. — Ostodolepis , a primitive 

 theromorph reptile. Vertebrae from in 

 front and side, with primitive double- 

 headed rib and intercentrum. 



