SUB-KINGDOM VERTEBRATA. 



centra ; but by the disappearance of the latter they have frequently 

 become transferred to the centra or their appendages, although in 

 other cases their original derivation is indicated by their articulation 

 at the junction of two vertebrae. Very generally ribs articulate with 

 the vertebral column by two distinct heads, which is probably the 

 original primitive type of structure. In these cases the anterior or 

 lower head is termed the capitular, and articulates with the rib- 

 facet or parapophysis on the vertebral centrum (fig. 815, cp) ; while 

 the posterior or upper head is known as the tubercular, and in the 

 dorsal vertebras articulates with the transverse process or diapophysis 



of the arch. In the dorsal ribs the 

 two heads may, however, coalesce, 

 and articulate with a facet on the 

 centrum, or with a longer or shorter 

 transverse process on the arch. Fre- 

 quently, moreover, while the anterior 

 dorsal ribs have double heads, the 

 posterior ones have but a single head 

 articulating with the transverse pro- 

 cess. In the caudal region of many 

 Reptiles true ribs articulate with the 

 upper part of the centrum, and ex- 

 cept as being separate, are indis- 

 tinguishable from the caudal trans- 

 verse processes of many Mammals, 

 such as the Cetacea (fig. 817). 

 Cervical ribs are present in most 

 Reptiles, and usually articulate by 

 two heads to the vertebrae — the 

 upper head joining a facet, generally 

 placed on the arch, corresponding 

 to the transverse process of the dorsal vertebrae, and the lower one 

 articulating to another facet on the centrum. Very rarely in Rep- 

 tiles these cervical ribs may be completely anchylosed to the ver- 

 tebrae, as in the vertebra of Calamospondylus, figured in the sequel 

 (fig. 107 1) ; and this suggests that at least a portion of the so-called 

 transverse processes of the cervical vertebrae of Mammals, which 

 arise from the vertebrae by double pedicles, really correspond to 

 cervical ribs. In the Sauropterygia both heads of the cervical 

 ribs articulate with the centrum. 



When a sternum is present the distal ends of the dorsal ribs are 

 generally unossified, and are sometimes termed intermediate ribs 

 (ir, fig. 819); and these unossified elements unite distally with the 

 sternal ribs (sr), which in their turn join the sternum (fig. 817, b, b). 

 Further, among the Sauropsida lateral ossifications may be developed 



Fig. 819. — Sternal region of a young 

 Ornithorhynchus. c, Clavicle ; i, Inter- 

 clavicle ; p, Presternum ; ins, Mesoster- 

 num; r, Rib; ir, Intermediate rib; sr, 

 Sternal rib. (After Flower.) 



