CHAPTER III 



THE RIBS AND STERNUM ^ 



The ribs of reptiles, like those of the amphibians, primitively articu- 

 late with all vertebrae, at least as far back as the middle of the tail. 

 The first to become fixed or closely united with the vertebrae, after 

 the sacral, were the caudal, next the lumbar, and last of all the cervi- 



FiG. 86. Vertebrae and ribs: A, Clidastes (Mosasauria), posterior cervical vertebra, from 

 behind; B, Cymbospondylus (Ichthyosauria), anterior dorsal vertebra, from the side, after 

 Merriam; C, Ichthyosaurus (Ichthyosauria), middle dorsal vertebra, from the side (after 

 Broili); D, Dimetrodori (Theromorpha), anterior dorsal rib; E, Diadectes (Cotylosauria), 

 anterior dorsal rib. 



cal. The dorsal ribs are free, except in the Chelonia, some Ptero- 

 sauria, and some armoured dinosaurs. 



The ribs of the Temnospondyli (Fig. 86) articulate with intercen- 

 trum and arch, usually without differentiation of the articular sur- 

 faces. And this was the original mode among reptiles. With the 

 diminution in size of the intercentrum, the head, or capihilmn, joins 

 the adjacent ends of two centra across the intervertebral cartilage, 

 the articular surface, however, continuous to the tubercle, which 

 articulates with the end of the diapophysis. This continuous articu- 

 lation from the intercentral space to the arch was the almost invari- 



1 [For the morphology and variation of the ribs, in connection with the segmentation 

 of the vertebrate body, see Butschli, 1921, Vorlesungen iiber Vergleich Anat.; Kingsley, 

 Compar.-Anat. Cert. — Ed.] 



