8 PEOFESSOE OWEN OS THE CHAEACTEES, ETC. 



mouth is closed. The other teeth of the first set are the ' deci- 

 duous molars ; ' the teeth which displace and succeed them verti- 

 cally are the ' premolars ; ' the more posterior teeth, which are not 

 displaced by vertical successors, are the ' molars ' properly so called. 



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,' and four may be true 

 ' molars ;' or there may be four premolars, and three true molars. 

 This difference, as I have elsewhere shown, forms a character of a 

 secondary group or order in the mammalian class*. The essential 

 nature of the distinction is as follows : true molars are a backward 

 continuation of the first series of teeth ; they are developed in the 

 same primary groove of the foetal gum; they are 'permanent' 

 because they are not pushed out by successional teeth — the ' pre- 

 molars,' called ' dents de remplacement ' by Cuvier. Seven teeth 

 developed in the primary groove is, therefore, the typical number 

 of first teeth, beyond the canines. If, as in Didelphys, the 

 anterior three develope tooth-germs, which come to perfection in 

 a ' secondary groove,' there are then 3 deciduous teeth, 3 pre- 

 molars, and 4 true molars : if, as in Gymnura^ the anterior four 

 of the ' primary ' teeth develope tooth-germs, which grow in a 

 secondary groove, there are then 4 deciduous teeth, 4 premolars, 

 and 3 true molars. The first true molar of the marsupial is thus 

 seen to be the liomologue of the last milk-molar of the placental. 



The Gymnure, the Mole, and the Hog are among the few exist- 

 ing quadrupeds which retain the typical number and kinds of 

 teeth. In a young Hog of ten months, the first premolar, p.l, 

 and the first molar, m. 1, are in place and use together with the 

 three deciduous molars, d. 2, d. 3, and d. 4 ; the second molar, 

 m. 2, has just begun to cut the gum ; p. 2,^. 3, and^. 4, together 

 with m. 3, are more or less incomplete, and will be found concealed 

 in their closed alveoli f. 



The last deciduous molar, d. 4, has the same relative superiority 

 of size to d. 3 and d. 2, which m. 3 bears to m. 2 and m. 1 ; and the 



* Outlines of a Classification of the Mammalia, Trans. Zool. Soc. vol. ii. p. 330 

 (1839). 



t I recommend this easily acquired * subject ' to the yoimg zoologist for a 

 demonstration of the most instructive pecuharities of the mammaUan dentition. 

 He will see that the premolars must displace deciduous molars in order to rise 

 into place : the molars have no such relations. 



