1893.] MR. M. r. WOODWAKD OK MAMMAXilAIf DENTITIOTf. 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 we should restrict the number of the latter to 

 four, when in so many of these fossils and even amongst the 

 living forms, viz. Mijrmecohius (where no replacing tooth is 

 known), we find that there may be 5 or 6 of the anterior cheek- 

 teeth 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 cheek- 

 tooth, and has on this account restricted the number of the pre- 

 molars 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 

 Phascolof/ale. 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 Uke Amphitylus and Dryolestes is 

 to be interpreted in this Hght. 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. giganfeus being the only one which shows anything like rudi- 

 ments 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). May 



Peoc. Zool. Soc— 1893, No. XXXn. 32 



