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Show 1893.] MR. M. F. WOODWARD ON MAMMALIAN DENTITION. 469 amongst those most lowly animals the Mesozoic Mammalia, rather than amongst the highly modified Placentalia? If we turn to the published accounts of these old mammals, we find that many of them possessed a large number of cheek-teeth, which were in some cases divisible into two series : thus, in Amphilestes, where there were 12 or 13, the anterior six can be separated from the posterior ones by their simplified structure. In many cases, however, no sharp line can be drawn, as the anterior teeth become gradually more complicated as we pass backward. It is important to note that in the reduction in the number of these teeth which takes place in some (as in Phas-colotherium), it is obvious that this has not occurred so much at the posterior end of the series, but rather that certain teeth in the middle have been either completely suppressed or retarded, thus reducing the number of teeth, especially in the premolar region. Supposing the cheek-teeth in the Marsupials all belong to the same set, either the 1st or 2nd dentition, then the only difference between the molars and premolars comes to be one of form ; and I see no reason why w e should restrict the number of the latter to four, when in so many of these fossils and even amongst the living forms, viz. Myrmecobius (where no replacing tooth is known), we find that there may be 5 or 6 of the anterior cheekteeth of simpler character than those behind. The so-called successor to the 4th premolar I regard as one of these anterior teeth (possibly the 4th or the 6th) which has been retarded in its development, and, by the backward growth of the tooth in front and the forward growth of the tooth behind, has assumed a position underneath these teeth, and has consequently to displace one of them in order to reach the surface. Thomas has shown the presence of this tooth in Triconodon under the 4th cheektooth, and has on this account restricted the number of the premolars to four in these early Marsupials ; but this form possesses a dentition in which the number of teeth is already greatly reduced, only possessing 7 or 8 cheek-teeth, and consequently differing very little from the least modified Marsupials such as Phascologale. There is, I. believe, no evidence to show that this condition had been acquired by forms like Amphilestes, with numerous cheek-teeth, unless what appears to be a retardation of the 5th cheek-tooth in forms like Amphitylus and Bryolestes is to be interpreted in this light. This tooth, however, is regarded as the 1st molar in these forms. As I have already pointed out, the evidence adduced for the development of the premolars is not decisive enough to settle definitely to which dentition these teeth are to be referred, M. giganteus being the only one which shows anything like rudiments of a second set. If they belong, as Kiikenthal suggests, to the 1st dentition, then I should be inclined to think that Baume's (1) theory as to the formation of the permanent teeth was not so far wrong after all (of course I leave out of consideration the Cetacea, for which Kiikenthal has definitely proved the contrary). M ay PROC. Z O O L . Soc-1893, N o . X X X I I . 32 |