8 MAMMALIA. 



possess teeth invariably planted in distinct sockets in the jaw ; in most 

 cases the young animal has inilk teeth, which are ultimately succeeded by 

 perinancnt teeth. In man the teeth may be divided into four groups: the 

 incisors or cutting teeth in front, the canine or eye-teeth, the false molars, 

 and the back teeth or molars. The number of teeth in any animal is 

 usually expressed by what is called the " Dental Formula." We know 

 that in each half of each jaw there is a like number of teeth ; hence we 

 have in man the " dental formula" as follows: 



I. ^-^^ C. i^, F. M. ^^, M. ^^ = 32; 



2 — 2' I — I 2 — 2 3 — 3 



where \. denotes incisors ; C, canine ; Y.M.., false molars ; y\.., true molars ; 

 the figures above the line the number of teeth in the upper jaw ; those in 

 the lower line, in the lower jaw ; the first numbers in each group, the 

 teeth in the left ; the second ones, the teeth in the right jaw ; the final 

 figures, the total number of teeth in the mouth. The same dental formula 

 is given for the chimpanzee, but what a difference it presents to that of 

 the ruminant or cud-chewing animal ! The dental formula of the sheep, 

 for example, is 



I. 5-=^, C. ^=^, F. IM. ^^, M. 3-=^ = 32. 



Thus we see at a glance that while the sheep has as many teeth as our. 

 selves, they are widely different from ours in position. In the upper jaw 

 the incisors and canines are wanting ; the false molars are three in each 

 side, while we have but two ; the molars in both jaws are as numerous 

 as our own ; in the lower jaw there are three incisors on each side 

 against our two, and the same number of canines as we possess. 



In the Carnivora, or flesh-eating animals, the molars assume a cutting 

 character, while in those that feed on herbage, the Herbivora, the molar 

 structure prevails. In the Rodcntia, or gnawing animals, such as rats or 

 squirrels, the incisors project forward and are continuallv growing ; in 

 the elephant there are no lower incisors, but the upper incisors, two in 

 number, grow into enormous tusks. In the adult whale, the teeth are 

 replaced by the whalebone plates. 



The digestive organs of the Mammalia do not differ to any great 

 extent. They possess one stomach with the exception of the Runiinantia, 

 or cud-chewers, which have four, the first three of which are so arranged 



