272 ELEMENTARY ANATOMY. [LESS. 



We see by this, that there may be teeth with no vertical pre- 

 decessors in front of others which have such predecessors. 



Milk-teeth may be entirely absent in some Mammals, as is 

 the case in the Porpoises and Dolphins and most Edentates. 



The canine teeth, as in the highest Apes, may appear after 

 the true molars are in place. Generally the last deciduous 

 molar resembles, not its vertical successor, but the first true 

 molar, as has been stated with regard to man. 



A mode of succession may exist in man's class which 

 renders it difficult to define the essential nature of the anterior 

 molars. Such is the case in the Elephant, where six teeth 

 are successively developed, the hindmost ones being much 

 more complex than the anterior ones. Each rotates into its 

 position in such a way that one portion of it is worn before 

 the last part is in place, and ultimately the whole of the jaw 

 is occupied by a single tooth. The Elephant has, however, 

 undoubtedly two upper deciduous incisors on each side, and 

 three molars ought in all probability to be reckoned as 

 deciduous ones on each side of each jaw. 



Below Mammals we may find quite other conditions of 

 replacement and succession ; but though of course there is 

 never a "milk" set, yet even in some Fishes there is a 

 vertical succession like that occurring in Mammals. This, 

 for example, is the case in the teeth of the front of the mouth 

 of the Fish Sargus which teeth, moreover, bear a singular 

 resemblance to the incisors of man. 



In the Crocodiles, as the teeth wear out or become lost, 

 they are replaced by others, formed on the inner side of 

 those which they are destined to succeed. Each new tooth 

 causes, by its development, an absorption of the inner wall 

 of an old tooth, and, entering at this aperture, it becomes 

 enclosed within the central cavity of the latter, so that when 

 the old tooth is removed the new one is found rising up 

 in its place. This process appears to go on indefinitely 

 through the whole of life in these creatures. 



A far more abundant supply of new teeth and ready re- 

 placement of old ones are found in the Sharks. Whole rows 

 of teeth, getting ready for use, lie folded back one behind the 

 other all round the jaws. As the old teeth are lost, those 

 immediately behind go forward, become erect, and take the 

 vacant places. 



The most singular succession of all is found in the Parrot 

 Fishes (Scams). Parrot Fishes browse upon those truly 

 sensitive plants the arborescent polyps, and their jaws are 



