TEETH 



323 



The Marsupials form a marked contrast to those Mammals re- 

 ferred to on p. 319 in which the milk-dentition has undergone re- 

 duction, for in them there is only one functional successional tooth, 

 the fourth premolar, all the others belonging to the milk series. 

 Bud-like enamel-germs occur, however, near all the persistent 

 teeth anterior to the third molar, and these must be looked upon 

 as rudiments of potential teeth and not as vestiges which have 

 become functionless. 



In Ornithorhynchus, in which there are indications that two 

 dentinal series are represented, the three multitubercular teeth 

 present on either side of the upper and lower jaws become replaced 

 functionally after a time by the development of horny masticatory 



FIG. 237. OUTLINE OF THE SKULL OF GEOMYS, SHOWING THE RELATIONS OF THE 



TEETH (after V. Bailey). 



i, incisors (which in Rodents correspond to the second incisors of other Mammals) ; 



p*, premolars ; in, 1,23, molars. 



plates, 1 and in Echidna they are wanting altogether, the dental 

 lamina undergoing reduction at an early stage. Adult Whalebone- 

 Whales and certain Edentates (Myrmecophaga, Manis) are toothless, 

 but vestiges of teeth occur in the embryo. In other Edentates the 

 teeth are wanting in enamel. Canines are absent in certain 

 Mammals (e.g. Rodents), and the incisors may also be wanting. In 

 the typical Ruminants functional incisors and canines are present 

 in the lower jaw only, though tooth-vestiges occur in the pre- 

 rnaxillary region of the embryo and occasionally upper canines persist 

 in the adult. 2 



1 Horny crushing plates are also present in the Sirenia, the existing forms of 

 which possess numerous teeth, while the extinct Rhytina was toothless. 



2 In the Manatee, the dentition is very peculiar. Traces of incisors, canines, 

 and premolars occur in the embryo, but the adult possesses molars only, which 

 undergo a constant succession from behind throughout life as the anterior ones 

 are pressed forward and fall out ; there may be as many as 8 10 functional at 

 the same time. In the Elephant there is also a similar succession of molars, but 

 it is here limited to six. 



Y 2 



