TOOTH-DEVELOPMENT IN OKNlTHOliHYKCEUS. 159 



suppression. Of the five teeth which we take to be the full 

 complemeut of members of the quasi-permanent series of 



V 



Avhich we have evidence, we hold the first and second , ^ 



(v) 



" w " 

 and to correspond to premolars. This judgment is 



based largely upon the relative size and simplicity of the 



'^ w '^ . . 



tooth -, which is the best developed of these (?) pre- 

 molars, in both the earlier and later stages at our disposal. 

 The proportions may be judged of by the schemes in 

 text-figs. 1 and 2, and the form by a comparison of fig. 3 with 

 figs. 1 and 2. 



" w " 

 If the tooth - — y, be premolar, then the molar formula may 



be expressed, in accordance with our views and nomenclature, as 



<f X y — (z) " 



— -^j' There can be no hesitation in identifying 



X y z 



these teeth as molar. 



But if we are also right in our recognition of the " nodular " 



series, including the obvious denticle '' dy^," as vestigial 



. "dv" 



teeth in series with the undoubted vestigial teeth j—^ — r, 



dv 



then we have before us in Ornithorhyuchns a demonstiation 

 of the presence of a whole series of precursors of the molar 

 teeth. And it is not the least remarkable aspect of this 

 demonstration that it exhibits these vestigial deciduous pre- 

 decessors of the large molar teeth in the form of a much more 

 numerous series of simple tooth-rudiments, each on the whole 

 corresponding with one of the cusps of their multicuspidate 

 molariform successors. The admission that there are excep- 

 tions to this general correspondence of deciduous nodule to 

 permanent cusp, as seen in text-fig. 2, can hardly succeed in 

 invalidating the idea of a deep-seated and definite corre- 

 spondence which is otherwise suggested by the facts. It is 

 not very difficult to imagine some explanation of the duplicity 



"dyi — dy2" 

 of the nodules ttji a 2 ?> *^^^ ^^^® lines of a theory of the 



