xra PHYLUM CHORDATA 299 



hoops of bone, twice as numerous as the arches, and alternately 

 dorsal and ventral in position. The former represent centra, the 

 latter intercentra, or ossifications alternating with the centra on 

 the ventral region of the notochord. 



The first or cervical vertebra bears paired articular surfaces for 

 the condyles of the skull, and between them the anterior face of 



Mh -^ 



Fig. 946. — Longitudinal 'sections of vertebral centra of A, Ranidens; B. Amblystoma ; 

 C, Spelerpes ; and D, Salamandrina. Ch. nptochord ; GK, intra-vertebral cartilage an d 

 fat-cells ; Gk, convex anterior face of centrum ; G'p, concave posterior face ; Jvk. inter-vertebral 

 cartilage ; K, superficial bone of centrum ; Ligt, inter- vertebral ligament ; Mli, marr6w-cavity ; 

 R, transverse process ; S, intra-vei"tebral constriction. (From Wiedersbeim's Comparative 

 Anatomy.) 



the centrum gives off, in Urodela, a projection called the odontoid 

 process. The Urodela, moreover, have ribs articulating with the 

 transverse processes of the abdominal and sacral vertebrae : they 

 are short bones, forked proximally, and the compressed transverse 

 processes are correspondingly divided. The sacral ribs of Urodeles 

 give attachment to the ilia, and the caudal vertebrae bear heemal 

 arches. 



The sJmll of Urodela differs from that of the Frog in many 



