TOOTH-DEVELOPMENT IN ORNITHORHTNCHUS. 153 



fiud them exemplified in the majority of instances {cf. fig. 12) 

 The more posterior of the two nodular structures, however, 

 differs in a highly significsmt manner from the general type of 

 nodule. Its structural characters are well shown in the 

 photomicrographs in figs. 9 and 11. It will be seen that 

 within a concentrically arranged connective-tissue capsule 

 there is a pale zone, which on high-power examination 

 (fig. 1 1) shows radially arranged cells identical with those of 

 the internal epithelium of the main enamel-organ as shown in 

 fig. 9. Within this epithelium is a deeply stained ring of 

 dentine, and within this, again, is a connective-tissue core or 

 pulp. We have, therefore, here present all the essential 

 structures of a typical tooth, and there can be no hesitation in 

 regarding it as a vestigial tooth. Yet in its position and 

 relations to the enamel-organ, whose superficial aspect it 

 slightly indents, it exactly resembles the concentric epithelial 

 nodules with which it is in series [cf. figs. 8 and 9, and 11 

 and 12). 



In the case of the lower teeth (text-fig. 2) quite similar 

 relations obtain in respect of the presence of epithelial nodules 

 in the vicinity of the principal cusps, which are in this case 

 the external cusp series. Here, again, there are two nodules 

 which are more or less in the neighbourhood of the anterior 

 chief cusp of " y." The more posterior of these is placed in 

 advance of the prominence of the cusp with which it is pre- 

 sumably associated. In fact, it lies just in front of the 

 anterior limit of calcification of the cusp, instead of over its 

 prominence. The second and more anteriorly placed nodule 

 iu this region is much smaller and more superficially placed, 

 and it lies wholly in front of the calcified anterior cusp of 

 the tooth " J." Indeed, so far as its position is concerned it 

 is placed rather in a vertical relation to the hinder portion of 

 the enamel-organ of the more anterior tooth " x,'^ though 

 behind the plane of the posterior calcified external cusp of 

 the latter. The idea of its possible relation to the tooth " x " 

 is further supported by the fact that it lies more labially than 

 the larger nodule behind it, which is related to the anterior 



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