TYPICAL OR ANCESTRAL SET OF TEETH 139 



eight grinders, four canines, and twelve incisors is not 

 found complete in many mammals at the present day, 

 though it is found more frequently as we go back to 

 earlier strata.^ 1 Though some mammals have kept close to 

 the original number, they have developed peculiar shape 

 and qualities in some of the teeth as well as changes in 

 size. The common pig still keeps the typical number 

 (Fig. 15). But he has developed the corner teeth or 

 canines into enormous tusks both in the upper and lower 

 jaw, and the more anterior grinders have become quite 

 minute. The cats (lions and tigers included) have kept the 

 full number of incisors (see figs. 26 and 27, pp. 160, 161) ; 

 they have developed the four canines into enormous and 

 deadly stabbing " fangs," and they have lost all the 

 grinders but three in each half of the lower jaw and 

 four in each half of the upper jaw (twelve instead of 

 twenty-eight), and these have become sharp-edged so as 

 to be scissor-like in their action, instead of crushing or 

 grinding. Man and the old-world monkeys have lost 

 an incisor in each half of each jaw (see Pis. VII and VIII) ; 

 they retain the canines, but have only five molars in each 

 half of each jaw (twenty in all instead of twenty-eight). 

 Most of the mammals whatever change of number 

 and shape has befallen their teeth in adaptation to their 

 different requirements as to the kind of food and mode of 

 getting it have retained a good long pair of jaws and a 

 snout or muzzle consisting of nose, upper jaw, and lower 

 jaw, projecting well in front of the eyes and brain-case. 



* Mammals having the number and form of teeth which I have just 

 described as typical or such modification of it as can easily be produced 

 by suppression of some teeth and enlargement of others are called Typi- 

 dentata. On the other hand, the whales, the sloths, ant-eaters, and arma- 

 dilloes, as also the Marsupials, are called Variodentata, because we cannot 

 derive their teeth from those of the Typidentate ancestor. They form lines 

 of descent which separated from the other mammals before the Typidentate 

 ancestor of all, except the groups just named, was evolved. 



