MASTICATION. 251 



In birds teeth are never present as organs of mastication, their 

 place being taken by the muscular gizzard; but the horny coating of the 

 jaws is developed in successive laminaj, especially seen in the parrot, 

 which forms the beak, and which serves, in the prehension of food, the 

 same purpose as the prehensile tooth of other animals. 



In the mammal the greatest variety is met with in the number, 

 the shape, and external characteristics of teeth. They may vary in 

 number from one in the narwhal to as many as one hundred and ninety 

 in the dolphins. In the elephant there are at most ten, but usually only 

 six, namely, one entire molar, or sometimes parts of two on each side 

 of both jaws, together with the two tusks of the upper jaw. In the 

 rodents the ordinary number is twenty, but there are sometimes only 

 twelve, while in the hare and rabbit there are twenty-eight. In rumi- 

 nants and commonly among the mammalia there are thirty-two ; but 

 forty-four (as in the hog and mole) is said by Owen to be the typical 

 number. When more than forty-four teeth are present, as is occasion- 

 ally the case in the lowest groups, they are of the reptilian type, as in 

 the porpoise. 



Three kinds of teeth, as already mentioned, are met with: the in- 

 cisors, which are chisel-shaped for cutting and gnawing ; the canines, 

 which are longer and conical for tearing food; and the premolars and 

 molars, which are variously cusped and tuberculated, and either flat- 

 tened at the sides for cutting or broad at the summits for grinding. 

 The incisors are smallest in the insectivora, larger in the carnivora, of 

 great strength in the herbivora, and especially strong in the rodents. 

 These vary in number : the lion has six in each jaw ; the squirrel two 

 highly-developed incisors in each jaw ; the ruminants none in the upper 

 jaw; the elephant none in the lower jaw; while the sloth has none at all. 

 The canine teeth are prominent, conical, and larger than the other teeth 

 in the dog and cat tribes ; but not so in man. They are also large in 

 many non-carnivorous animals, as in the ape, bear, musk-deer, and others 

 where they are used as weapons of offense and defense. There are 

 never more than four, and are wanting in rodents and most herbivora. 

 The carnivorous molars are generally flat, ridged, or tuberculated, the 

 anterior ones being, as a rule, very small ; they overlap like the blades 

 of a scissors, and are, therefore, cutting and not grinding teeth in these 

 animals. The more purely carnivorous the species, the fewer the number 

 of molars. The herbivorous molars are provided with tubercles, as in 

 the quadrumana, man, and most omnivora, or are marked with trans- 

 verse ridges of enamel and dentine in the ruminants, solipedes, pachy- 

 dermata, and rodents. The premolar teeth are preceded by milk-teeth; 

 the true molars have no predecessors. In mammals the teeth are con- 

 fined to the jaw-bones, fit closely in the sockets, may have one or more 



