19 



therefore, the typical number of first teeth, beyond the canines. 

 If, as in Didelphys (fig/ 2), the anterior three develope tooth- 

 germs which come to perfection in a ' secondary groove,' there 

 are then 3 deciduous teeth, 3 premolars, and 4 true molars : 

 if, as in Sus, fig. 3, 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 mo- 

 lars. The first true molar of the marsupial (fig. 2, m. 1, d. 4), is 

 thus seen to be the homologue of the last milk-molar of the 

 placental (fig. 3, d. 4). 



The Hog, the Mole, the Gymnure and the Opossum, are 

 among the few existing quadrupeds which retain the typical 

 number and kinds of teeth. In a young Hog of ten months 

 (fig. 3), the first premolar, p. 1, 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, p. 3, and^>. 4, together with m. 3, are more 

 or less incomplete, and will be found concealed in their closed 

 alveoli 1 . 



The last deciduous molar, J.4, has the same relative supe- 

 riority of size to d. 3 and d. 2, which m. 3 bears to m. 2 and 

 m. 1 ; 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. When the milk-teeth are shed, and the permanent 

 ones are all in place, their kinds are indicated, in the genus 

 Sus, by the following formula : 



. 33 1 1 44 33 



*-^> c -r=T' p-4=i> -3^ =44 >' 



which signifies that there are on each side of both upper and 

 lower jaws 3 incisors, 1 canine, 4 premolars, and 3 molars, 

 making in all 44 teeth, each tooth being distinguished by its 

 appropriate symbol, viz. p. 1 to p. 4, m. 1 to m. 3. This number 

 of teeth is never surpassed in the placental diphyodont series. 



1 I recommend this easily acquired ' subject' to the young zoologist for a 

 demonstration of the most instructive peculiarities of the mammalian dentition. 

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

 into place : the molars have no such relations. 



C 2 



