HOMOLOGIES OF TEETH. 377 



p 2, and the further additions in lower quadrupeds, until in the 

 Hog genus we see the old primitive type of diphyodont dentition 

 resumed or retained. 



In the genus Sus, fig. 293 illustrates the phenomena of de- 

 velopment which distinguish the premolars from the molars. At 

 the stage exemplified the first premolar, 1 p l, and the first molar, 

 m i, are in place and use, together with the three deciduous 

 molars, d 2, d3 9 and d 4 ; the second molar, m 2, has just begun 

 to cut the gum ; p 2, p 3, and p 4, together with m 3, are more or 

 less incomplete and concealed in their closed alveoli. 



The premolars displace deciduous molars in order to rise into 



294 



Deciduous and permanent teeth (Sus). Lower jaw 



place ; the molars have no such relations ; it will be observed, 

 that the last deciduous molar, d 4, has the same relative supe- 

 riority of size to d 3 and d 2 which m 3 bears to m 2 and m l ; and 

 the crowns of p 3 and p 4 are of a more simple form than those of 

 the milk-teeth which they are destined to succeed. 



The premolars have a more simple structure as well as smaller 

 size, than the true molars, in all Artiodactyles. In the Ru- 

 minants they represent only the moiety of the true molars, or 

 one of the two semi-cylindrical lobes of which those teeth consist, 

 with, at most, a rudiment of the second lobe. The Perissodactyles 

 are distinguished by the size and complexity of more or less of 

 •the premolars. In Equus, p 2, p 3 and p 4, even exceed in size 

 m l, ni2 and rn 3. In Rhinoceros and Palceotherium the propor- 

 tions of the molars and premolars are reversed ; but the struc- 

 ture is the same. In Lophiodon, Coryphodon and Pliolophus the 

 premolars become more simplified as well as diminished, ap- 



1 If this tooth have not displaced a minute milk- molar, it may be reckoned a d I, 

 which is longer retained than the rest of the deciduous molars ; in this degree the 

 type-dentition is departed from. 



