OF THE CLASS MAMMALIA. 7 



to the jaws of the adult, as the ' temporary ' teeth were adapted 

 to those of the young animal. 



Those permanent teeth, which assume places not previously oc- 

 cupied by deciduous ones, are always the most posterior in their 

 position, and generally the most complex in their form. The term 

 'molar' or 'true molar' is restricted to these teeth. The teeth 

 between them and the canines are called ' premolars ; ' they push 

 out the milk-teeth that precede them, and are usually of smaller 

 size and simpler form than the true molars. 



Thus the class Mammalia, in regard to the times of formation 

 and the succession of the teeth, may be divided into two groups, 

 monopJiyodonts*, or those that generate a single set of teeth ; and 

 the diphyodontsi, or those that generate two sets of teeth. But 

 this dental character is not so associated with other organic cha- 

 racters as to indicate natural or equivalent subclasses. 



In the Mammalian orders with two sets of teeth, these organs 

 acquire fixed individual characters, receive special denominations, 

 and can be determined from species to species. This individuali- 

 zation of the teeth is eminently significative of the high grade of 

 organization of the animals manifesting it. 



Originally, indeed, the names 'incisors,' 'canines,' and 'mo- 

 lars,' were given to the teeth, in Man and certain Mammals, as 

 in Eeptiles and Fishes, in reference merely to the shape and offices 

 indicated by these names ; but they are now used as arbitrary 

 signs, in a more fixed and determinate sense. In some Carnivora, 

 e. g. the front-teeth have broad tuberculate summits, adapted for 

 nipping and bruising, while the principal back-teeth are shaped 

 for cutting, and work upon each other like the blades of scissors. 

 The front-teeth in the Elephant project from the upper jaw, in the 

 form, size and direction of long pointed horns. In short, shape 

 and size are the least constant of dental characters in the Mam- 

 malia ; and the homologous teeth are determined, like other parts, 

 by their relative position, by their connexions, and by their 

 development. 



Those teeth which are implanted in the premaxillary bones, and 

 in the corresponding part of the lower jaw, are called 'incisors,' 

 whatever be their shape or size. The tooth in the maxillary bone, 

 which is situated at or near to the suture with the premaxillary, 

 is the ' canine,' as is also that tooth in the lower jaw, which, ir, 

 opposing it, passes in front of the upper one's crown when the 



* fiovos, once ; ^vw, I generate ; odoiif^ tooth. 



t Sis, twice ; ^vu) and odovs. See " Philosophical Transactions," 1850, p. 493. 



