724 MAMMALIA. 



The typical mammalian dentition already referred to may be expressed 

 as follows : 



Incisors 3^2, canines ^ ^, premolars 4H4 } molars 3H3 = II j* 1 = total, 44 ; 

 33 i i 44 33 ii it 



or, using initial letters 



i. 3=3 c . !Z5, pm . =4 m . 3=3 ^ 



33' I i' F 44 33 



or, recognising that the right and left side are almost invariably identical, 

 and omitting the initial letters ^^. 



The formulae for the adult dentition of some representative Mammals 

 are the following : 



Opossum 5 -^, Thylacine ^_ 4 , Kangaroo 3 -^ f Wombat ^, Pig 3 -^, Camel 



4134 3134' 1024' 1014' ^3143' 31 



Sheep 2223, Horse 2I Rabbit ^ 3 , Cat ^ Dog ^> Bear ^ Seal ^ 

 3133 3i43 1023' 3121' & 3143 3143' 2141 



Hedgehog , Marmoset ^^, New World Monkey 1 , Old World Monkey ^, 

 & 2123' 2132' * 2133' J 2123' 



Man 2 -!H3. 



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 slippery 

 and struggling 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 

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

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

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

 is used ; when the cusps form a crescent, the tooth is called selenodont ; 

 when there is a long crown with the neck (the junction region between 

 crown and root) deep in the socket, the tooth is called hypsodont ; 

 when there is a short crown with the neck at the surface of the gum, the 

 term brachyodont is used. 



Development and placentation. The ova of Mammals, 

 except Monotremes, are small ; even those of the Whales are 



