MAMMALIA. 313 



" perinaiieut " teeth. The deciihums and pemianent sets of teeth 

 do not necessarily correspond to one another ; but no Mammal has 

 ever more than these two sets. The Mammals with two sets of 

 teeth are called by Owen "diphyodont." 



In Man and many other Mammals the teeth are divisible into 

 four distinct groups (fig. 224), which differ from ojie another in 

 position, ajjpearance, and function ; and whicli are known respec- 

 tively as the incisors^ canines^ p)xrnnolars, and molars. 



"Those teeth which are iniplaiiteil in the proemaxniary bones, and in the 

 corresponding jiart of tlie 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 swture with the prs^maxillary, is the 'canine,' as is also that tooth in 

 the lower jaw which, in opjtosing it, passes in front of its crown when the 

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

 the teeth which displace and succeed them vertically are the ' pra;molars ' ; 



Fig. 224. —Teeth of the right side of the lower jaw of the Chimpanzee (after Owen\ 

 i Incisors ; c Canine tooth ; pm Pra^niolars; m Molars. 



the more posterior teeth, which are not disjdaced hy vertical successors, are 

 the ' molars' properly so called " (Owen). The deciduous dentition, therefore, 

 of a diphyodont Mammal consists of only three kinds of teetii — incisors, 

 canines, and molars. The incisor and canine teeth of the deciduous set are 

 replaced by the teeth wliicli hear the same names in the permanent set. The 

 deciduous "molars," however, are replaced by the permanent " prpemolars," 

 and the *' molars " of the permanent set of teeth are not represented in tlie 

 deciduous series, only e.\:ist!n,? once, and not being replaced by successors. 



All these kinds of teeth are not necessarily present, and the teeth 

 furnish most important characters fctr separating the various orders 

 of Mammals frcmi one amtther. For tliis reason it is usual to 

 express the number of the teeth in any jfarticular animal by an 

 arithmetical formula, called the dciitol formrUi. For example, the 



