16 MAMMALIA class v 



much earlier period than in the members of the same families in early 

 Tertiary times. 



The number of teeth in monophyodont mammals is highly variable, 

 following no fixed law. Primitive diphyodont mammals have 44 teeth in 

 the permanent dentition, the dental formula being If, C ^, P -^, M -|. In the 

 milk dentition the formula is DI f , DC i DM f . Among diphyodont forms 

 not only are the teeth of the right and left jaws symmetrically arranged, ' 

 but there prevails a definite law as to the number and position of the diflPerent 

 teeth. In almost every order and family of mammals those occurring in the 

 more recent geological formations have fewer teeth than the early representa- 

 tives of the order or family ; while an increase above the normal number 

 rarely occurs in any of the diphyodont mammals. The reduction almost 

 invariably begins at one or the other end of any series of teeth, for instance 

 with the last or first incisor, or with the first premolar, or the last molar, 

 and proceeds from that point either forward or backward.^ 



The form and size of the different teeth depend on their function, position 

 and nourishment. One can safely say that the conical, single-rooted tooth set 

 at regular intervals along the jaw was the primitive type of the ancestral 

 mammal reptiles, and the canine tooth represents the least deviation from 

 this type. The canines are generally conical, single-pointed teeth (two-rooted 

 only among certain marsupials and insectivores), used chiefly for tearing and 

 grasping the food, and therefore chiefly developed among carnivores. 



The incisors are used as a rule for biting off and grasping the food : if 

 for grasping, they are conical and similar to canines ; if for biting, the crown 

 is compressed from front to back, making a cutting chisel- or gouge-like edge. 

 In all cases they are single-rooted. In case the incisors serve for special 

 functions, they become modified accordingly. Thus, those employed for 

 gnawing become large, slightly curved, sharp, usually long and very frequently 

 with enamel only on the anterior surface. The strong, conical upper incisors 

 of the Sirenia are adapted for uprooting aquatic plants ; the massive tusks, with 

 persistent pulp, and either entirely lacking enamel or having it in bands as in 

 the case of the proboscidians and of narwhals, serve as weapons, or for the 

 removal of obstacles, and the like. AVith increase in size and specialisation of 

 the incisors a decrease in number is apt to occur. Complete lack of these is 

 found in the Edentata ; ruminants 1-ose the upper incisors entirely, and most 

 proboscidians the lower ones. 



The cheek-teeth serve by far the greatest variety of functions, and 

 accordingly they show the greatest amount of specialisation. As a rule, they 

 display a tendency for the crown to become enlarged and provided with 

 tubercles, cusps or ridges for the purpose of masticating food. Carnivores and 

 insectivores use their cheek-teeth entirely for cutting up their prey and 

 crushing the bones. Teeth with narrow, elongated, pointed and multi- 



^ 111 making the abbreviated dental formula, the teeth of a single jaw only are used, the 

 expression, for example, of the tapir's dentition being thus written : — 



,-1, permanent dentition, if, c{-, p|, mf = |-j'^ 2»=:44, 



or shorter, |, ^, ^, |. 

 B, milk or deciduous dentition, dif, dc^, dm§, =f x2 = 28, 



or shorter D = f, y, %. 



When reductions have occurred they appear in the formula, as for example in the case 

 of the goat : — 



n s 09 



