XIII 



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 inter centra, 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 



-Li 3 t 



Fio. 946. Longitudinal "sections of vertebral centra of A, Ranidens ; B. Amblystoma ; 

 C, Spelerpes ; and D, Salamandrina. Ch. notochord ; C'tf'.intra- -vertebral cartilage and 

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

 cartilage ; K, superficial bone of centrum ; Ligt. inter-vertebral ligament ; Mh, marrow-cavity ; 

 X, transverse process ; S, intra-vertebral constriction. (From Wiedersheim'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 vertebra? bear ha3mal 

 arches. 



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



