138 J. T. WILSON AKD J. P. HILL. 



stage, or perhaps very slight!}^ younger; tlie otlier belongs 

 to a distinctly earlier period. 



In this coranninication we do not propose to enter into any 

 detailed discussion of the original literature dealing with the 

 dentition of Ornithorhynchus. This, indeed, consists merely 

 of Poulton's two papers (1, 2), and of papers by Mr. Oldfield 

 Thomas (4) and Professor Charles Stewart (5), on the 

 characters and condition of tlie erupted teeth in the adol- 

 escent animal. Sucli points as require attention will receive 

 special notice in the course of the paper. 



The material which Professor Poulton had at his disposal 

 seems to have represented three distinct specimens from the 

 collection of Professor W. K. Parker, but of these two were 

 rejoresented only in a fragmentary way. All the specimens 

 would seem to have belonged to the same period of develop- 

 ment. 



In the stage described by him Poulton found that there 

 were present in the upper jaw 'Hhree considerably developed 

 and large teeth." These were already ranlticnspidate, Avith 

 the principal cusps calcified. In addition to these, and 

 immediately behind and to the inner side of the posterior 

 one, he found another'tooth-rudiment in a very early stage of 

 development. 



In the lower jaw he found corresponding tooth-structures, 

 except that corresponding to the anterior tooth of the upper 

 jaw. The actual presence or absence of the lower opponent 

 of the latter could not be definitely ascertained owing to 

 imperfection of the material, bub its existence was inferred 

 as highly probable. The presence of four teeth was thus 

 regarded as established, with certainty in the upper jaw, and 

 with great probability in the lower. 



Subsequently Oldfield Thomas (4) and later Charles Stewart 

 (5) described the appearance of the fully-developed teeth in 

 the half-grown animal. From Stewart's description and 

 figures it appears that three teeth are erupted in the lower 

 jaw, of which the anterior two are very large and multi- 

 cuspidate, whilst the posterior one is very much smaller. 



