162 J. T. WILSON AND J. P. HILL. 



In this connection we may note that we cannot now admit 

 the validity of the furthei* surmise of the same writer that 

 the concentric epithelial cell-nests formerly described by us 

 as occurring in Perameles are of the same order or signi- 

 ficance as those we describe and figure in Ornithoi'hynchus. 

 In our previous paper (8, p. 518) we have ourselves expressed 

 the opinion, subsequently endorsed by Tims, as to their resem- 

 blance to the nodules of Ornithorhynchus. This, however, 

 was prior to our study of the latter. We are now perfectly 

 confident that the cell-nests we encountered in Perameles 

 belong to a totally different category of structural differentia- 

 tion. These latter are purely epithelial degeneration products 

 formed in, or in connection with, the mouth epithelium over- 

 lying teeth which are about to undergo ei'uption ; they are 

 quite irregular in their occurrence and in their arrangement; 

 they make their appearance, for the first time, in comparatively 

 late stages of tooth-development — i.e. shortly before erup- 

 tion, and they contain at no time any trace of structural 

 elements save the concentrically arranged epithelial cells 

 themselves, having no elements referable to an origin from 

 the mesoderm as is the case with the genuine " nodular " 

 structures in Ornithorhynchus. They are, in fact, entirely 

 similar to the epithelial " pearls " which are met with in 

 various situations, as, for example, in the neighbourhood of 

 the median raphe of the palate ; whereas we have shown that 

 the nodules in Ornithorhynchus when fully developed show 

 obvious traces of the typical structure of an enamel-organ. 



A further conclusion from the view of tooth-development 

 and tooth-homology in Ornithorhynchus set forth by us is 

 that no support is to be derived from it for any theory which 

 would seek to establish a serial distinction between the true 

 molar teeth and those which appear, prima facie, to belong 

 to the same series in front of them. It is quite true that the 

 suppression of the antemolar teeth has pi'oceeded so far that 

 it would be rash to base any theory of serial homologies 

 between molars and antemolars on the condition obtaining 

 in Ornithorhynchus. All that we here affirm is that, so far 



