17 



single set of teeth, and the DipJiyodonts 1 or those that generate 

 two sets of teeth. But this dental character is not so asso- 

 ciated with other organic characters as to indicate natural or 

 equivalent sub-classes. 



In the Mammalian orders with two sets of teeth, these 

 organs acquire individual characters, receive special denomi- 

 nations, and can be determinated from species to species. 

 This differentiation of the teeth is significative of the high 

 grade of organization of the animals manifesting it. 



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

 ' molars,' were given to the teeth, in Man and certain Mam- 

 mals, as in Keptiles and Fishes, in reference merely to the 

 shape and offices indicated by those 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 tubercu- 

 late 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. Indeed, shape and size are 

 the least constant of dental characters in the Mammalia ; 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' (fig. 2, i), 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 (ib. c), which, in opposing it, 

 passes in front of the upper one's crown when the mouth 

 is closed. The other teeth of the first set are the ' deciduous 

 molars ' (d. 1 3) ; the teeth which displace and succeed them 

 vertically are the ' premolars ' (p. 1 3) ; the more posterior 



1 Sis, twice; <t>vw and oSoi's. See Philosophical Transactions, 1850, p. 493. 



C 



