270 COMPARATIVE ANATOMY OF THE TEETH. 



confined to those possessing a spine. In the invertebrata, 

 instead of true teeth, the parts answering this purpose are 

 rather horny or calcareous indurations. 



The class mammalia, with but few exceptions, have teeth ; 

 and, according to M. Geoffrey, St. Hilaire, some animals 

 which appeared to be entirely without them, were found to 

 possess them during a portion of their life. 



Every tooth in the human subject consists naturally of 

 a crown, neck and root; but this division does not apply 

 to all the mammalia, for the incisors of the rodentia, and 

 the tusks of the elephant, are covered with enamel over 

 their whole extent, and are hence said to be without roots. 

 This distinction is, however, not considered good, since por- 

 tions of these teeth are imbedded like the roots of all others 

 in alveolar cavities. 



The teeth of the mammalia are divided into " simple 

 compound, and semi-compound or mixed/ 5 



Like the human teeth, the simple have no " anfractuosi- 

 ties" on their outer surface. The crown consists of a regu- 

 lar shell of ivory, covered with a smooth and even layer of 

 enamel. 



The compound teeth, on the other hand, look like several 

 teeth joined together, as they have their surface presenting 

 such deep sinuosities, and the cavity of the compound tooth 

 has as many subdivisions as there are parts joined together. 

 "A good idea," says M. Cuvier, "of the compound teeth of 

 animals may be drawn from the human molar teeth, which 

 have a simple crown and compound root, whilst the former 

 have generally a simple root and compound crown. Sup- 

 pose the roots of the large human molars, covered with 

 enamel and joined together by cement, and you have a type 

 of the compound teeth of other mammalia." -f* 



The teeth of the mammalia are also divided, like the 

 human teeth, into the deciduous or milk, and the permanent 

 teeth. The number of teeth in this class varies very much, 

 though less than in the other vertebrata. The highest 

 number is stated to be 190, and only to be found in the 

 dolphin. The form of the teeth constitutes the especial 



