664 MAMMALIA. 



adult, and not of ontogenetic importance. Thus in many cases, notably 

 in Ungulates, the permanent molars cut the gum, and come into use 

 long before any of the milk-teeth are shed. This fact not only makes 

 it doubtful to which set the molars should be ascribed, but also shows 

 that the members of two sets may function simultaneously. It is thus 

 apparent that in any given case we cannot assert that because all the 

 teeth in a skull have been functional at the same time, they necessarily 

 all belong to the same set. Further, one or both sets may be aborted, 

 and, apart from the molars, certain of the teeth may remain un-re- 

 placed. A macroscopic examination of the skull can therefore never 

 determine to which set particular teeth belong. According to Leche, 

 the one test which can be relied on is that the germs of the members of 

 a dentition are differentiated simultaneously, or almost simultaneously, 

 from the dental lamina. The test is obviously one that is not very 

 easy to apply, and the following account must be received as merely 

 tentative : — 



In the first place, we may notice that the question to which dentition 

 particular teeth or teeth rudiments belong, is one of considerable 

 theoretic importance. Mammals are typically diphyodont, reptiles are 

 polyphyodont, but Mammals arose from a reptilian stock, therefore one 

 would naturally hope to find that the lowest Mammals showed traces of 

 a polyphyodont condition. Of the teeth of the Monotremes little is 

 known, they occur only in the young of Oiiiithorhynchus. The 

 Marsupials have numerous well -developed teeth, and it has long been 

 known that only one functional tooth is replaced — the last pre- 

 molar. More recently it has been shown that in connection with 

 certain of the other teeth there occur rudiments of precociously 

 calcified teeth. The condition of the teeth in Marsupials was therefore 

 quite recently described as follows : — Marsupials are potentially 

 polyphyodont, for they show traces of three dentitions. Of these the 

 first is in process of suppression, and is represented by " pre-lacteal " 

 rudiments ; the second or milk dentition is important and functional, 

 but tends to diminish in importance in the Eutherian Mammals ; the 

 third is represented in Marsupials by the last premolar only, but in 

 most Eutheria becomes the functional dentition of the adult. Still 

 more recent work has, however, cast doubt upon this theory, and 

 brought the Eutheria and Metatheria into much closer relationship with 

 one another. According, to Wilson and Hill, the vestigial teeth 

 ("prelacteal" rudiments) of Marsupials are milk-teeth homologous 

 with the milk premolar, and the permanent teeth of Marsupials are 

 homologous with the second set of other Mammals. These authors 

 therefore believe that Marsupials, no less than Eutherian Mammals, 

 are primitively diphyodont, and that in Marsupials in general the milk- 

 teeth are in process of suppression, just as they are also in certain of the 

 Eutheria (seals, Bradypus, Erinaceus). This, taken in conjunction 

 with the discovery of a true allantoic placenta in Perameles among 

 Marsupials, tends to show that both Marsupials and Placentals must 

 have arisen from a primitively proto-placental and diphyodont stock. 

 In the Marsupials the placenta has become degenerate or aborted, the 

 milk diet has become increasingly important, and, in adaptation to 

 it, the young have lost or are losing the milk-teeth. In the Eutheria 



