HOMOLOGIES OF TEETH. 379 



d 5, and only becomes a permanent molar because there is no 

 premolar developed above it, so we may regard the tooth marked 

 m i in figs. 221-230 as being an antecedent tooth of the deciduous 

 series, rendered permanent by a like reason, the suppression, viz. 

 of p 4. In other words, that m i in fig. 227 is the homologue of 

 d 4 in fig. 294, and that the true homologue of p 4 is not deve- 

 loped in the Marsupialia. 



The homologies of the teeth of the Kangaroo are illustrated in 

 fig. 296, according to this idea of them ; the dental formula of 

 both the MacropodidcB and Hypsiprymnida being - 



. 3J5. ^ Ll 1.1. / L1 - 3.3 



instead of 



.3.3 1.1 1.1 4.4 

 i ; c ; p ; m - = 30. 

 1.1' 0.0' 1 1.1' 4.4 



The canines, which are confined to the upper jaw, are small or 

 minute when retained ; and disappear after being represented ' en 

 o-erme ' in most of the true Kangaroos. 



o o 



In the deciduous dentition of the great Kangaroo (Macropus 

 major) the canines are rudimental, and are absorbed rather than 

 shed. No other of the deciduous series is calcified, save the 

 molars d 2 and d 3, fig. 296, unless the permanent incisors be de- 

 veloped and retained milk-teeth. When the young animal finally 

 quits the pouch the dentition is- 



j-' 1 - 2 ' 2 



the upper incisors being i i, the molars d 2 and d 3 of the typical 

 dentition. This stage is exemplified in the lower jaw at A (fig. 

 296). The next stage shows the acquisition of i 2 in the upper 

 jaw, and d 4 in both jaws, and the formula is 



2.2 3.3 



d i ; d m -^ = 18, ib. B. 

 l.i. o.o 



At one year old, the dentition is 



3.3 3.3 1.1 



a i ; a m ; m = z ; 



1.1 O.Q 1.1 



the additional teeth being i 3 and m i (ib. c), in which the demon- 

 stration of the true deciduous character of d 2 and d 3 is shown 

 by the germ of their vertical successor p 3, which is exposed in 

 the substance of the jaw. The next stage is the shedding of 

 d 2, and the acquisition of m 2 (ib. D). Then d 3 is shed by 



