TOOTH-DEVELOPMENT IN ORNITHOKHYNCHUS. 147 



are constrained to regard the relatively elongated enamel- 

 organs which are posterior iu both jaws of the younger 

 stage as the representatives^ not of the teeth " y '' and " y/' 

 but of "x" and "x^' respectively in the jaws of the older 

 stage, " Beta." It will be observed that botli in the younger 

 and in the older "x" is substantially more elongated than 

 " x," and is also situated iu a somewhat more advanced posi- 

 tion. 



(C „ » 



The above identification of ^^ — y, in the younger specimen 



necessitates the assumption that, between this stage and the 

 older one there has occurred a tolerably rapid growth and 

 differentiation of the posterior portions of the dental lamina 

 of the, younger stage [v. text-fig. 1). This is precisely what 

 one might expect to take place during the process of differ- 

 entiation of a molar series ; and it has already been shown 

 that, not only is the hinder segment of the dental lamina in 

 the earlier stage relatively plump and well-developed, but 

 that there is actually present in it the early rudiment of an 

 actual posterior tooth. Further, it may be stated that the 

 increase iu absolute length of the head which has occurred 

 in the later stage is very considerable, allowing of marked 

 elongation of the posterior region of the jaw and of the molar 

 lamina. And, in fact, comparative measurements show that 

 the total length of the lamina in the older stage is not merely 

 absolutely but relatively greater than in the younger. 



Comparison of the two schemes in text-figs. 1 and 2 (which, 

 though schematic, are nevertheless drawn to scale) will show 

 that in the older specimen the enamel-organ " x" is not only 

 larger than its upper opponent, but is actually the largest 

 tooth-rudiment in either of the jaws. On the other hand, the 

 lower anterior enamel organ, " w," in the younger specimen, 

 is appreciably smaller than its upper opponent, " w," and this 

 early inferiority of " w " already foreshadows the marked 

 reduction of "w," as compared with " w," in the older foetal 

 specimen, as well as its eventual entire absence in the 

 adolescent animal. 



