8l6 PHYLUM CHORD ATA I CLASS MAMMALIA 



connection between the teeth and the subjacent bones is a secondary 

 matter ; there is often httle to differentiate canine from premolar. 

 Moreover, the teeth of the lower jaw, which is a single bone on each 

 side, cannot be so certainly classified as those of the upper jaw. Here 

 the lower canine is defined as the tooth which bites in front of the 

 upper, and the incisors as the teeth in front of this tooth. 



The typical mammalian dentition already referred to may be expressed 

 as follows : — 



Incisors ^—^, canines ^-^, premolars llZl^ molars ^^^ = ^^ ^^ = total, 44 ; 

 3—3 I— I 4—4 3—3 II— II 



or, using initial letters — 



. ^ — ^ I — I 4 — 4 3 — 3 



1. ^ — ^, c. , pm. ^ — ^, m. ^ — ^ = 44 ; 



3—3 I— I 4—4 3—3 



or, recognising that the right and left sidesare almost invariably identical, 



and omitting the initial letters — ^-~^. 



The formulae for the adult dentition of some representative Mammals 

 are the following : — 



Opossum 5134, Thylacine 1134, Kangaroo 3_I^. Wombat £211, Pig 3113^ Camel 1113. 

 ^ 4134 3134 1024 1014 3143 3123 



Sheep °233, Horse 3113, Rabbit ^^33, Cat 313i, pog ^ll^, Bear ^B3, Seal liH- 

 3133 3143 1023 3121 3143 3143 2141 



Hedgehog 3133, Mannoset ^^32, ^ew World Monkey ^^^, Old World Monkey ?i^- 

 2123 2132 2133 2123 



Man ^^. 

 2123 



It is interesting to note the relation in particular cases between the 

 diet and the form of the teeth. Thus the dolphins, which feed on fish 

 and swallow them whole, have numerous, almost uniform, sharp, 

 recurved, conical teeth, well suited to take a firm grasp of the shppery 

 and strugghng booty. To a slight extent the same piscivorous dentition 

 may be seen in seals. In the more strictly carnivorous Mammals the 

 incisors are small, the canines are long and sharp, piercing the prey 

 with a deathful grip, while the back teeth have more or less knife-like 

 edges, which sever flesh and bone. In typical insectivorous Mammals 

 the upper and lower incisors meet precisely, " so as readily to secure 

 small active prey, quick to elude capture but powerless to resist when 

 once seized," while the crowns of the molars bear many sharp points. 

 Herbivorous Mammals have front teeth suited for cropping the herbage 

 or gnawing parts of plants, the canines are small or absent, the molars 

 have broad grinding crowns with transverse ridges. In omnivorous 

 Mammals the incisors are suited for cutting ; the canines are often 

 formidable weapons in the male sex ; the molars have crowns raised 

 into rounded tubercles. 



A primitive form of tooth with three cusps in one plane is called 

 Iriconodont ; when the three cusps form a triangle, the tooth is called 

 trituhercular ; when the crown has a number of blunt or pointed cusps, 

 it is called bunodont ; when the cusps run into ridges, the term lophodont 



