[Illustration: Fig. 10.--The teeth in the upper and lower jaw-bone of the common pig--drawn from photographs. A and B represent the right half of the lower jaw (A) and the right half of the upper jaw (B) seen in horizontal position. _Inc._ are the incisors or chisel-like front teeth, three in number, in each half of each jaw and marked 1, 2, 3. _C_ marks the canine or dog-tooth, which here grows to be a large tusk. The molars, "grinders," or cheek teeth are marked 1 to 7. Figs. C and D give a side view of the left halves of the upper (C) and of the lower jaw-bone (D), with the teeth in place. The bone has been partly cut away so as to show the fangs or roots of the teeth, which are double in the molars, and even threefold in molar No. 7. The explanation of the lettering is the same as that given for Figs. A and B. The letter _p_ in Fig. B points to a "foramen" or hole in the upper jaw-bone. These drawings are introduced here as showing the _complete_ number of teeth which the ancestor of pigs, goats, elephants, dogs, tigers, men, and even whales possessed. The reduction in number and the alteration in the shape of the primitive full set of teeth is referred to in the present chapter on "Elephants," and in those on "Vegetarians and their Teeth" (p. 102), and on "A Strange Extinct Beast" (p. 92).] Now this typical set of teeth--consisting of twenty-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.[6] 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. 10), 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. 21 and 22, pp. 103, 104); 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 Pls. VI and VII); 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. Man is remarkable as an exception.