18 



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

 'molars' properly so called (in. 1 4). 



Fig. 2. 



Lower Jaw of a young Opossum (Diddphys). 



I have been led, chiefly by the state of the dentition in 

 most of the early forms of both carnivorous and herbivorous 

 Mammalia, which flourished during the eocene tertiary periods, 

 to regard 3 incisors, 1 canine, and 7 succeeding teeth, on each 

 side of both jaws, as the type formula of diphyodont dentition. 



Three of the seven teeth may be 'premolars' (fig. 2, p. 

 \ 3), and four may be true 'molars' (ib. in. 1 4) ; or there 

 may be four premolars (fig. 3,^>. 1 4), and three true mo- 

 lars (ib. m. 1 3). This difference forms a character of an 



Lower Jaw of a young Pig (Sus.) 



ordinal group in the mammalian class l . The essential nature 

 of the distinction is as follows: true molars (ib. m.) are a 

 backward continuation of the first series of teeth (ib. d.) ; they 

 are developed in the same primary groove of the foetal gum ; 

 they are 'permanent' because they are not pushed out by the 

 successional teeth (ib. >.), called 'dents de remplacement ' by 

 Cuvier. Seven teeth developed in the primary groove is, 



1 Outlines of a Classification of the Mammalia, Trans. Zool. Soc. Vol. n. 

 !' 330 



