MAMMALIA. 313 



" permanent " teeth. The deciduous and permanent 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 one another in 

 position, appearance, and function ; and which are known respec- 

 tively as the incisors, canines, prcemolars, and molars. 



" Those teetli which are implanted in the priemaxillary 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 praemaxillary, is the ' canine,' as is also that tooth in 

 the lower jaw which, in opposing 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 ' praemolars ' ; 



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

 i Incisors ; c Canine tooth ; pm Prajinolars ; vi Molars. 



the more posterior teeth, which are not displaced by vertical successors, are 

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

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

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

 replaced by the teeth which bear the same names in the permanent set. The 

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

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

 deciduous series, only existing once, and not being replaced by successors. 



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

 furnish moat important characters for separating the various orders 

 of Mammals from one another. For this reason it is usual to 

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

 arithmetical formula, called the dental formula. For example, the 



