TOOTH-DEVELOPMENT IN OENITHORHYNCHUS. 145 



upper jaw, bearing in miud tlie fact tliat in both of our stages 

 the entire dental hiniina and its derivatives in the lower jaw 

 are uniformly in advance, as regards position, of the corre- 

 sponding structures in the upper. This small tooth, then, we 

 indicate by the symbol "w." Now, some distance in front 

 of the tooth '^w" in the upper jaw the deeper portion of the 

 dental lamina shows a small and rudimentary, but distinctly 

 papillated, enamel-organ. This we designate as ^^v." It 

 lies a short distance in front of the plane of "w" of the lower 

 jaw. No trace of any lower opponent to "v" is to be found. 

 It is, however, obvious that the anterior region of the dental 

 lamina in Ornithorhynchus is that which is most under 

 the influence of those factors which have brought about the 

 special modification of the mouth and jaws. It is, accord- 

 ingly, in this region that we might expect to meet with 

 evidences of suppression. And since, as we have already 

 pointed out, the dental lamina in the lower jaw is topo- 

 grajjhically in advance of that of the upper jaw, it is not 

 surprising that no trace of a lower "v^' is to be met with in 

 this relatively late stage. 



Passing now to the posterior region of the dental lamina of 

 our older specimen, we find that the posterior extremity of 

 the tooth "y" projects behind the plane of its opponent 

 "j." Behind the latter the dental lamina shows a small, but 

 well-developed, papillated enamel-orgau, uncalcified, and 

 just beginning to emancipate itself from the residual dental 

 lamina. This enamel-organ we here designate as "z." 

 Opposite to this in the upper jaw we still find the hinder 

 extremity of "y.'^ When the posterior end of the latter is 

 reached, the upper dental lamina continues for a short 

 distance (0'15 mm.) as a thick and bulky structure and then 

 suddenly stops without showing any definite differentiation 

 into an enamel-organ. This terminal thickened portion of the 

 lamina may, however, be regarded as the aniage of a potential 

 upper "z," and Poulton would appear to have found it 

 actually papillated, although it apparently never erupts. 



