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 1 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 Macropodidce and Hypsiprymnidce being — 



instead of- 



.3.3 1.1 1.1 ,1.1 3.3 QA 



.3.3 1.1 1.1 4.4 on 



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

 minute when retained ; and disappear after being represented i en 

 germe ' in most of the true Kangaroos. 



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 — 



,.1.1 J 2.2 mtk 

 dl TTV dm 2.2 = l2; 



the upper incisors being i l, 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 — 



J .2.2 . 3.3 , „ 

 a i — ; dm— =18, lb. b. 

 1.1 6.6 



At one year old, the dentition is — 



3.3 , 3.3 1.1 OA 



the additional teeth being i 3 and m l (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 



