CHAP. VI] THE DENTAL SYSTEM OF THE PRIMATES 275 



Orang-utan (v. ante Figs. 172 and 173), evidence of a transition in 

 type from one class to another. Such animals as present these 

 variations in their normal tooth forms are described as heterodont, 

 and are hereby distinguished from homodont animals, the latter 

 possessing teeth of one pattern only ; and usually it is found that 

 homodont teeth are present in large numbers disposed along an 

 elongated jaw. 



Again, the animals in which replacement of a deciduous set of 

 teeth by a permanent series has been observed, are referred to as 

 diphyodont, in distinction from monophyodont animals, in which 

 but one set of teeth is seen. The Primates (including Man), 

 and indeed most of the higher mammals, are heterodont and also 

 diphyodont. 



The Metatheria (Marsupials) appear to be monophyodont, but 

 it has long been known that their most posterior premolar tooth 

 is replaced : moreover Leche, Hill, and Wilson have discovered 

 traces of other teeth which do not attain functional perfection. 

 The Prototheria provide some evidence in the same direction, and 

 in fact it is stated by Tims (./. A. and P. vol. xxxv., p. 321) that 

 no mammal has yet been shewn to be absolutely monophyodont. 

 Moreover, in addition to the two sets of teeth so characteristic of 

 the Eutherian mammals, vestiges have been discovered of two other 

 series, which may be described as constituting a " pre-milk " and 

 a " post-permanent " set respectively. This discovery, which we 

 owe to the work of Rose and Leche, has increased the complexity 

 of the problem of deciding which sets of teeth are homologous in 

 mammals of the various Sections : and in particular, the identifi- 

 cation of the ordinary set of teeth of Marsupials is still a matter 

 of controversy. It is also to be remarked that according to the 

 respective researches of Tims and Rose, the human foetus possesses 

 unmistakeable vestiges of representatives of both pre-milk and 

 post-permanent series, and it is a characteristic feature of the 

 abortive tooth-germs that the so-called pre-lacteal germs should 

 be developed on the labial side of those germs which proceed to 

 become functionally active, while the post-permanent representa- 

 tives are placed on the lingual side of the permanent teeth. 



In his exhaustive article in the Bibliotheca zoologica (Heft 17, 

 1895, Stuttgart), Leche remarks that in the four dentitions (which 



18—2 



