54 TEETH OF CAPE ANTEATER CHAP. 



same phenomenon ; for Sir W. Flower showed, and Mr. Thomas 

 confirmed his discovery, that only one tooth, according to Mr. 

 Thomas the fourth premolar, is replaced in that group. But 

 even the purely monophyodont dentition of the Toothed Whales 

 is a more apparent than real contrast to the diphyodont dentition 

 elsewhere prevalent. An investigation of the embryos of various 

 Toothed Whales by Dr. Kiikenthal and by Dr. Leche has brought 

 to light the highly important fact that two dentitions are present, 

 but that one only comes to maturity ; from this fact obviously 

 follows the interesting question : To which of the two dentitions 

 of more normal Mammalia does the monophyodont dentition of the 

 Whales and Marsupials belong ? To this question a clear answer 

 is fortunately possible. As has been pointed out in the fore- 

 going sketch of tooth development, and has been illustrated in 

 the figures, the milk teeth develop as lateral outgrowths of the 

 common enamel germ, while the permanent teeth arise from the 

 end of the same band of tissue. This fact enables it to be 

 stated apparently beyond a doubt that in the Whales and in the 

 Marsupials it is the milk dentition which is the only one to 

 arrive at maturity. Thus the earlier theoretical conclusion that 

 the Marsupial dentition " is a secondary dentition with only one 

 tooth of the primary set left," is proved on embryological grounds 

 to be untrue. But there are other monophyodont animals than 

 those already mentioned. 1 Orycteropus, the Cape Anteater, is an 

 example. Mr. Thomas has lately discovered that in this Eden- 

 tate there is a set of minute though calcined milk teeth which 

 probably never cut the gum ; here we have a different sort of 

 monophyodontism, in which the teeth belong to the second and 

 not to the first set. Between the latter condition and the 

 diphyodont state are intermediate stages. Thus in the Sea Lions 

 the milk teeth are developed but disappear early, probably before 

 the animal is born. 



In the typical diphyodont dentition, such as is exhibited for 

 example in Man and the vast majority of mammals, the milk teeth 

 eventually completely disappear and are entirely replaced by the 

 permanent set of teeth, with the exception, of course, of the molars, 

 which though they are developed late belong to the milk series. 



1 It would be of the greatest interest 'in relation to this and many other 

 problems to ascertain the precise meaning of the monophyodont dentition of 

 Ornithorhynchus. 



