TOOTH-DEVELOPMENT IN ORNITHORHYNCHUS. 137 



Observations on Tooth-Development in 

 Ornithorhynchus. 



By 

 J. T. Wilson, 



Professor of Anatomy, University of Sydney, N.S.W., 



and 

 J. P. Hill, 



Jodrell Professor of Zoology, University College, London. 



With Plates 10—12. 



(1) Introductton. 



Since the appearance of Professor E. B. Poulton's first 

 announcement (1) of his discovery of true teeth of mammalian 

 type in a mammary-fcetal specimen of Ornithorhynclius, several 

 papers have appeared dealing witli the dentition of this animal. 

 Only one of these (2), embodying a more detailed account, 

 by Poulton himself, of his own investigations, concerns the 

 developing teeth prior to their eruption. The extreme 

 difficulty of procuring the necessary material has hitherto 

 stood in the way of further research in this direction. The 

 stage investigated by Poulton is accordingly the only one in 

 which, so far as we are aware, there is any record of the non- 

 erupted and immature dentition. 



The observations hei'ein set forth concern two specimens 

 of mammary foetus of Ornithorhynchus. One of these is 

 practically identical in its grade of development with Poulton's 



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