MAMMALS. 



367 



FIG. 158. Milk (shaded) and perma- 

 nent dentitions (outline) of the cat. 

 c, canines; pp 4 , premolars; m, 

 molar. The incisors (to left) not 

 lettered. From Boas. 



have teeth. These teeth are always confined to the 

 edges of the jaws (c/. 

 Fishes, p. 24), being in- 

 serted by one or more roots 

 into sockets in the bone. 

 Some mammals have but a W 

 single set of teeth through- 

 out life, but the majority 

 have a first or milk dentition, 

 which is soon lost and re- 

 placed by a permanent den- 

 tition. Occasionally, as in 

 the sperm-whale, etc., all the 

 teeth are similar in shape, 

 but usually several different 



kinds occur, the extreme being reached when four types 

 are present incisors, canines, premolars, and molars. 

 The incisors have but a single root, and are found in the 

 premaxillary bone and in the corresponding position in 

 the lower jaw. The first teeth in the maxillary, if single- 

 rooted and pointed, are called canines; and behind these 

 come the molars, with two or more roots. These in turn 

 are subdivided into premolars (the bicuspids of the dentist) , 

 which appear in both milk and permanent dentitions, and 

 molars proper, which occur only in the permanent set. 

 The number of teeth and their arrangement vary con- 

 siderably in different mammals, and the characters which 

 they furnish are of great value in grouping the various 

 species. To express these characteristics briefly a dental 

 formula has been introduced, in which the different kinds 

 of teeth are indicated by initials, while the number in 



\j 



each half of either jaw is represented by a figure above or 

 below a horizontal line. Thus the permanent dentition of 



