CLASS MAMMALIA 



425 



dentary to form a single bone which articulates directly with the skull, 

 and one has passed into the service of the ear (Sec. 407). The lower 

 jaws of the two sides are sometimes fused to form one mandible. Carti- 

 laginous discs separate the bodies of the successive vertebrae. The ribs 

 articulate both with the vertebrae and with the sternum and thus the 

 expansion and contraction of the thoracic cavity are made possible. 





Fig. 316. — Hair, magnified, showing the outer covering of the thin, horny, transparent, 

 plates of various forms. A, golden mole. B, red bat. C, beaver. D, woodchuck. E, 

 mink. F, skunk. G, cat. H, horse (Percheron). {Redrawn by Paul T. Gilbert from 

 Hausman, "Natural History," and the Sci. Man.) 



The teeth of mammals generally are represented by two sets, a milk 

 dentition and a permanent dentition. The teeth are set in sockets 

 in the jaws and are generally differentiated into several types, known 

 as incisors, canines, and molars. The character of the dentition shows 

 a specialization corresponding to the character of the food and the 

 manner of securing it. Each tooth consists largely of dentine (Fig. 318), 



