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Show 468 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, lamina situated immediately behind pm. 3, and morphologically in front of the 4th premolar. Further, if a skull be examined in which the tooth-change is taking place, it wdll 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 (fig. 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, w e find that the latter is invariably developed behind the former, and either cuts the g u m internal to it, as in the case of the incisors, or else comes up underneath it, but at the same time slightly internal and posterior. But in no case (unless the present instance in the Marsupials be one) does the permanent tooth develop and cut the g u m in front of its milk predecessor. Kiikenthal, 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 connection with the tooth behind it (pm 4), as I think it unlikely that these two forms should possess such striking differences in the development of their teeth. Unfortunately the specimens of Bidelphys 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 successional 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 aboATe, 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 explanation of the condition of these teeth in the Macropodidae 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 belief in the disappearance of the first two premolars in the Macropodidae to be the correct one, then w e 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 w e compare its form with that of 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 au explanation of order and succession of the Marsupials' teeth |