488 ilE. M. F. WOOUW VHD 0>' MAMMALIAN DEXTITIOy. [^MaV 2, 



lamina situated immediately behind pm. 3, and morphologically 

 in front ol: the 4th premolar. Further, if a skull be examined 

 in which the tooth-change is taking place, it will be seen that 

 the supposed successor of the 4th premolar cuts the gum in front 

 of that tooth to which it is believed to be its milk predecessor 

 (fie. 27). This would be quite an anomalous condition, for if we 

 study the relations of a milk-tooth to its permanent successor in 

 a typical placental mammal, we find that the latter is invariably 

 de\eIoped behind the former, and either cuts the gum internal 

 to it, as in the case of the incisors, or else comes up underneath 

 it, but at the same time slightly inteinaal and posterior. But in 

 no case (unless the present instance in the Marsupials be one) 

 does the permanent tooth develop and cut the gum in front of its 

 milk predecessor. 



Iviikenthal, it is true, figures what he regarded as the developing 

 functional successional tooth as arising from the 4th premolar. 

 If this structure really has the significance which he ascribes to it, 

 then I should suggest that he is probably dealing with one of 

 those modified conditions seen in the later stages of Petrogale, 

 in which the successional tooth has acquired a secondary con- 

 nection with the tooth behind it (jym 4), as 1 think it unlikely 

 that these two forms should possess such striking differences in 

 the development of their teeth. 



Unfortunately the specimens of Didelphys which I have at 

 present examined have been too old to show the earliest stage in 

 the formation of this tooth. 



The fact that the succe>sional tooth does actually replace the 

 4th premolar in these two forms is of course a strong argument 

 in favour of the older view that these two teeth represent the 

 milk and permanent stages of the 4th premolar : but still I think 

 the facts of development as described above, which suggest that the 

 older view is erroneous, cannot be ignored. Until, however, further 

 proof is forthcoming as to the development of these teeth in the 

 Polyprotodonts, it will not be wise to express too definite an 

 opinion on the matter ; but I nevertheless think the true explana- 

 tion of the condition of these teeth in the Macropodidse is that 

 this so-called successional tooth is not a successional tooth at all, 

 or at any rate to the 3rd and 4th premolar, but a tooth of the same 

 series intermediate in position between the two. 



Assuming the hehef in the disappearance of the first two pre- 

 molars in the Macropodidae to be the correct one, then we must be 

 here dealing with an animal possessing five teeth of the so-called 

 premolar series, the tooth which is generally regarded as the 

 successor to the 4th being itself the true 4th premolar, and the 

 so-called 4th premolar being in reality the 5th, or else the 1st 

 molar, which seems possible when we compare its form with that 

 oi the true molars. The presence of five premolars, considered in 

 the light of the dentition of the higher Mammalia, may seem to 

 be open to question. But should we not rather seek for an 

 explanation of order and succession of the Marsupials' teeth 



