538 



B. Howes for the opportunity of carrying out this work, and for much 

 kindly criticism and advice. 



Recent investigations into the mammalian tooth-genesis necessitate 

 caution in the usage of the terms Monophyodont and Diphyodont^ 

 Certain Marsupials , Edentates and Cetacea, formerly regarded as 

 Monophyodont, have, by the researches of Flower, Kükenthal and 

 others, been shown to be, in reality, Diphyodont, though only one 

 set of teeth may be functional. 



Not only is this the case, but evidence has been adduced in fa- 

 vour of the existence, among Mammals, in a very reduced condition 

 of two other sets of teeth, a so-called Pre-milk dentition, and what 

 may be termed a Post - permanent dentition; thus bringing the mam- 

 malian dentition more into line with the polyphyodont condition of 

 their reptihan ancestors. 



There are, however, certain teeth which are represented, in the 

 great majority of Mammaha, in a single dentition only. They are the 

 first premolar, and those of the molar series. The question naturalh 

 arises, to which dentition do they belong? 



Before attempting to give an answer to this question, it is ne- 

 cessary to examine the evidence upon which the existence of the four 

 above mentioned dentitions rests and to ascertain whether the facts 

 brought forward in support of it are capable of being otherwise inter- 

 preted. 



Let us consider first, the post-permanent dentition, that is, the 

 series represented by a downgrowth of the dental lamina on the 

 lingual side of the teeth of the permanent series. The evidence in 

 favour of the existence of these downgrowths is, I think, indisputable. 

 Leche(I) and Kükenthäl(2) have observed such traces in the Seal, 

 Rose (3) in Man, M. F. Woodward (4) in Erinaceus and I have found 

 clear evidence of its existence in the Dog. 



It is very different, however, in the case of the pre-milk den- 

 tition. In the first place it is necessary to remember that in no case, 

 as far as I am aware, have all four dentitions been shown to be pre- 

 sent in any one part of the jaw of any animal. Three, then, being 

 the maximum number present in any one region, it must be deter- 

 mined which are the dentitions represented. 



Leche (5) was the first to draw special attention to the presence 

 of tooth rudiments on the labial side of what he regarded as the 

 milk dentition in Myrmecobius. The fact of the existence of these 

 rudiments has been verified by Woodward (6) who also believes them 

 to exist in Phascologale and he has further shewn me that they are 



