265 



CHAPTER XXIX. 



DENTAL SYSTEM OF MAMMALIA. 



218. General characters of the Teeth. The present class in- 

 cludes a few genera and species that are devoid of teeth ; the 

 true ant-eaters (Myrmecophaga], the scaly ant-eaters (Manis), and 

 the spiny monotrematous ant-eater (Echidna), are examples of 

 strictly edentulous Mammals : Ornithorhynchus has horny teeth ; 

 the whales (Balcena, Balcenoptera) have transitory embryonic 

 calcined teeth, fig. 219, succeeded by whalebone substitutes, 

 fig. 217, in the upper jaw. The, female Narwhal seems to be 

 edentulous, but has the germs of two tusks in the substance of the 

 upper jaw-bones : one of these so remains ; the other becomes 

 developed into a large horn-like weapon in the male Narwhal, 

 fig. 220, A, and suggested to Linnaeus the name, for its genus, of 

 Monodon : but the tusk is never median, like the truly single 

 tooth on the palate of the Myxine ; and occasionally both tusks 

 are developed. In Hyperob'don the teeth are reduced in the 

 adult to two in number, whence the specific name, H. bidens ; 

 but they are very small and confined to the lower jaw. Ziphius 

 has two teeth of functional size and shape, one in each ramus of 

 the lower jaw ; and this is perhaps a sexual character. The 

 Delphinus griseus has five teeth on each side of the lower jaw : 

 but they soon become reduced to two. The Marsupial genus 

 Tarsipes is remarkable for the paucity as well as minuteness of 

 its teeth. The Elephant has never more than one entire molar, 

 or parts of two, in use on each side of the upper and lower jaws, 

 to which are added two tusks, more or less developed, in the upper 

 jaw. Some Rodents, Hydromys, e. g., have two grinders on each 

 side of both jaws, which, added to the four cutting teeth in front, 

 make twelve in all ; the common number of teeth in this order is 

 twenty ; but the hares and rabbits have twenty-eight teeth. 

 The sloth has eighteen teeth. The number of teeth, thirty-two, 

 which characterises man, the apes of the Old World, and the true 

 Ruminants, is the average one of the class Mammalia ; but the 

 typical number is forty-four. The examples of excessive number 

 of teeth are presented, in the order Bruta, by the Priodont 



