302 



VERTEBRATA. 



are usually amalgamated or " anchylosed " together to form a 

 single bone, the " sacrum " (s). 5. The spinal column is com- 

 pleted by a variable number of vertebrae which constitute the 

 "caudal" region, or tail (t). 



Fig. 269. Skeleton of the Beaver {Castor fiber), showing the different regions of the 

 vertebral column, c Cervical region ; d Dorsal region ; b Lumbar region ; s Sacrum ; 

 t Caudal region. 



As regards the skull of the Vertebrates, the most important 

 points to be noticed are the manner in which the cranium 

 articulates with the vertebral column, and the structure of the 

 lower jaw or " mandible." In Birds and Eeptiles the skull 

 articulates with the first vertebra of the neck by means of a 

 single articulating surface or " condyle," carried upon the 

 occipital bone. In the Amphibians, again, and in the Mam- 

 mals, there are two " occipital condyles," by which the skull 

 is jointed to the neck. The lower jaw is sometimes want- 

 ing, but, when present, it consists in all Vertebrata of two 

 halves or " rami," which are united to one another in front, 

 and articulate separately with the skull behind. In many 

 cases, each half, or " ramus," of the lower jaw consists of 

 several pieces united to one another by sutures ; but in the 

 Mammalia each ramus consists of no more than a single 

 piece. The two rami are very variously connected with one 

 another, being sometimes only joined by ligaments and mus- 



