158 J. T. WILSON AND J. P. HILL. 



K ^ }} 



III connection with tlie tooth <f ,, in the older specimen 



"Beta" we have seen that two nodules are present, " dx^ " 

 and " dx-," and that there was a possibility that the anterior 

 of the three nodules in relation with the enamel-oro-au of the 



lower tooth " y " is really related, to " x." There is not the 

 slightest doubt that there are multiple rudimentary nodules 



"x " 

 in connection with the hirge enamel-organ of 77 — r, in the 



younger specimen. But whether these are two or three in 

 number it is difficult to say. In the upper jaw Ave believe 

 that there are three, but the most posterior is at the hinder 

 extremity of the enamel-organ of "x," behind which we have 

 the thick dental lamina from which " y " subsequently arises, 

 so that we are unable to entirely exclude the possibility that 

 the last rudimentary nodule may really belong to "y." 

 We incline to the contrary opinion. There is abundant 

 evidence of the entire disappearance of some of the nodular 

 rudiments originally present in the fact that in the younger 

 specimen there appears to be two such differentiations of 

 typical character present in relation with the enamel-organ of 



the tooth , whilst this tooth in the latter stage is, as 



noted by Poulton, destitute of any vestigial nodule. 



(6) General Discqssion op Tooth-Homology in Ornitho- 



rhynchus. 



The facts set forth in the foregoing account seem to estab- 

 lish the existence in Ornithorhynchus of teeth belonging to 

 at least two dentitional series. In the series to which the 

 large multicuspidate teeth of the adolescent animal belong we 

 believe that we are justified in reckoning five members. In 

 the case of the upper jaw we lack positive evidence of the 

 actual formation of the most posterior member of the series, 

 and in the lower jaw, again, we believe that Ave are justified 

 in supposing that the most anterior member has undergone 



