XI. 



tooth on either side of each jaw, which is the predecessor of the 

 third and last pre-niolar. Only in the Anteaters (Myrmeco- 

 phagidtr), the Pangolins (Manidce), and the Echidna, are teeth 

 entirely absent at all stages of growth. 



SKELETON. 



Skeleton. The skeleton may be briefly denned as that portion 

 of the body of vertebrate animals which forms the framework on 

 which the muscles are supported. In the adult state the 

 greater part of this framework consists of osseous tissue, or bone, 

 the remainder being cartilage. Bone is mainly built up on a 

 gelatinous basis, strongly impregnated with salts of lime, chiefly 

 phosphate. After the teeth, the bones are the most imperishable 

 of all the organs of the body, and are, therefore, of great value in 

 affording reliable means of affixing the affinities of extinct with 

 recent forms. The skeleton is divided into two parts, the axial, 

 consisting of the skull and vertebral column, and the appendicular, 

 pertaining to the limbs. 



Skull. In the skull, or cranium, of adult Mammals, all the 

 bones, with the exceptions of the lower jaw, the auditory ossicles, 

 and the bones of the hyoid arch, are immovably articulated 

 together. The cranium, thus formed of numerous originally 

 independent ossifications, consists of a brain-case for the enclosure 

 and protection of that organ, and a face for the support of the 

 organs of sight, smell, taste, mastication, defence, and offence. 

 The brain-case articulates directly with the first cervical vertebra 

 by means of a pair of oval prominences, called condyles, placed on 

 each side of the large median foramen, which transmits the spinal 

 cord ; this method of articulation is termed dicondylian, and is 

 only present in one other class of Vertebrate Animals, the 

 BATRACHIA, and this, together with several other characters, com- 

 mon to these classes only, has given authority to the apparently 

 well-founded assertion of the remote common origin of the Mam- 

 malian and Batrachian types. 



Vertebral. The vertebral column consists of a series of distinct 

 bones, called " vertebrcc," arranged in close connection with one 

 another along the dorsal aspect of the body in the median line, and 

 extending from the posterior margin of the cranium (to which it 

 is firmly articulated) to the tip of the tail. The number of distinct 

 bones varies greatly, principally owing to the elongation, or other- 

 wise, of that appendage. In the mammalian vertebras the ends 

 of the centra are usually flattened, but in the cervical region of 

 some UNCULATA certain of the vertebra may be opisthoccelous, 

 that is having the hinder surface concave. The vertebral column 

 is for convenience divided into five regions, cervical, dorsal, 

 lumbar, sacral, and caudal. 



Cervical. The cervical region forms the anterior portion of 

 the column, and its first vertebra, called the atlas, articulates 



