TOOTH-DEVELOPMENT IN ORNITHOEHTNCHUS. 161 



An interpretation, practically identical with ours, of the 

 conceutric epithelial nodules and of their relations to, and 

 significance for, the molar dentition in Ornithorhynchus, has 

 been repeatedly advanced by Marett Tims (7). Thus on 

 p. 135 of his memoir on " The Evolution of the teeth in the 

 Mammalia " he says : " In the conceutric epithelial bodies 

 of Cavia, Canis, Gymnura, and Ornithorhynchus we have, I 

 believe, the last traces of a vanishing dentition which must 

 have preceded the cheek-teeth on account of their labial 

 position. These bodies remain quite distinct from the teeth 

 themselves and show no tendency to become fused." Again, 

 in reference to the more general question of molar fusion he 

 states (ibid.) that "an antero-posterior fusion of the teeth of 

 the same dentition appears to me now to be the only solution 

 of the difficulty in accounting for the duplex condition of the 

 true molars of the greater number of mammals and of the 

 complex cheek-teeth of the rodents and fossil multi-tuber- 

 culata. The repetition, so to say, of the development of the 

 anterior and posterior halves of the rodent molars seems to 

 me to render this highly probable, though I have not yet 

 seen any actual fusion of enamel-germs. It may quite well 

 be that this early stage may have become slurred over in the 

 I'ecapitulatory history, until it is entirely lost at the present 

 day. Possibly the same may be true of the ungulates and 

 proboscidians." 



So far as Ornithorhynchus is concerned, this writer had 

 only the data supplied by Poulton on which to base his 

 interpretation, and his judgment must have been determined 

 mainly by his experience of the occurrence of similar con- 

 centric epithelial bodies in the other forms specified. It is, 

 therefore, noteworthy that with our wider opportunities of 

 experience of the condition in the monotreme form we have 

 been led to conclusions which do not differ in any important 

 respect from those arrived at by Marett Tims. That view, 

 then, which for him could not in the nature of the case be 

 more than a surmise, as regards Ornithorhynchus, has through 

 our observations received more or less definite confirmation. 



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