TOOTH-DEVELOPMENT IN ORNITHORHYNOHUS. 155 



tion in botli jaws and on both sides of the jaw in our older 

 specimen " Beta." They would appear to have had a closely 

 similar arrangement in Poulton's specimen. 



{})) lu our specimen the typical concentric epithelial nodules 

 are in series with the undoubted vestigial toothlet which we 

 designate as "dy~"; and on one side of the upper jaw "dy-" 

 is represented by a typical concentric epithelial nodule. 



(c) Some of the typical nodules, though devoid of dentine, 

 show traces of other of the elementary tooth constituents. 

 Thus the nodule which we designate as " dy^ " shows traces 

 of a stellate enamel-epithelium around the condensed con- 

 centric laminas of epithelium; and several of the nodules 

 exhibit in their central core a collection of deeply-staining 

 nuclei similar to those which form the obvious pulp of the 

 genuine denticle ''dy~" {v. fig. 12, illustrating the structure 

 of a typical nodule). 



{d) The nodules appear to be identical in character with 

 those present in vai'ious other mammals which Tims has 

 shown to require, in all probability, a like interpretation. 



(4) Serial homology of the nodules with the 

 vestigial teeth "dv" and "dv" present in the 

 younger specimen " Delta." — In the coni'se of our 

 description of the younger of our two specimens, 

 " Delta," we have demonstrated the presence (figs. 4 and 

 5) of a vestigial tooth, "dv," in connection with the seg- 

 ment of the dental lamina lying in front of the more anterior 

 of the two large enamel-organs there present (i.e. our tooth 

 ''w"). We have previously given our reasons for regarding 

 this vestigial toothlet, " dv," as belonging to an earlier tooth 

 generation from that to which the more posteriorly placed 

 enamel-organs "w" and "x" belong. It now only remains 

 to indicate that in our opinion the strongly and precociously 

 calcified degenerative toothlet, ^' dv," is in series with, and 

 homologous to, the nodular vestiges present in the older 

 specimen in relation with the teeth " x " and " y." The 

 successor, " v," of the vestigial " dv " is represented in the 

 younger specimen by a swelling of the dental lamina which, 



