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THE POPULAR SCIENCE MONTHLY. 



the bone. The loss of an incisor results in the abnormal lengthening 

 of the opposing one, which may finally interfere with mastication and 

 cause starvation. Ruminants have no incisors in the upper jaw ; the 

 elephant has none in the lower jaw, but the tusks are upper incisors. 

 An elephantine mammal of former geological time, the dinotherium, 

 had incisor tusks in the lower jaw, pointing down and backward; while 

 the extinct mastodon had tusks in both jaws. 



The canines are intended for seizing and tearing prey and especially 

 characterize carnivorous mammals. They are lacking in rodents and 

 most herbivores, and are never more than four in number. In the 

 apes they are very prominent ; those of the gorilla nearly equaling a 

 lion's in size. The tusks of the walrus are upper canines, as also are 

 the terrible tusks of the wild hog. In the Malayan hog the upper 



Fig. 10. Babyrottssa, or Malayan Wild Hog. 



canines, instead of pointing downward, as would seem proper, grow up- 

 ward through the integuments of the skull and curve backward, some- 

 times reaching the skull again. Their purpose is obscure. Although 

 possessed in such degree by the male alone, the form precludes their 

 efficient use as weapons. The lower canines also grow to enormous 

 length and are directed outward, forming weapons which make the 

 beast a formidable antagonist. 



Herbivorous mammals have the molars flat on the grinding surface, 

 and the enamel and cement disposed in plates and folds perpendicular 

 to this face. Thus by the unequal wear of the tissues the acting sur- 

 faces are ridged and admirably adapted for grinding. They form an 

 actual grist-mill, the stones of which* never need any " picking." In 

 fruit-eaters the crowns of the teeth are rounded. Insectivorous mam- 

 mals have the teeth conical and fitting in opposing depressions. Those 

 living on a mixed diet, as man, have the tubercles or cusps somewhat 

 blunt, and suited either for crushing or cutting. In purely carnivorous 

 mammals the molars have sharp edges fitted for cutting meat. They 

 act like chopping-knives, or more accurately like scissors. Quite the 

 only motion of the jaw is up and down, as flesh can not well be ground. 

 Here we have a genuine "hash-mill." The backward and forward 



