TEETH OF PHYSETER 25 



lamina appears as a new structure. There is, so to speak, no 

 freeing of the residual lamina from the compound mass. 

 Strictly speaking, therefore, there is not an exact homology 

 between the concave surfaces of the future cup-shaped enamel 

 cap in the two series of teeth. It is terminal in one and lateral 

 in the other. 



But it must be remembered that, as I have pointed out above, 

 the more mature teeth, i.e. those at the symphysis end of the 

 jaw, apparently approach the teeth of the upper jaw in this 

 last-mentioned characteristic. The tooth germ, that is to say, 

 is more elongated and oval. But what we are dealing with 

 here is not the form of the growing tooth germ but its mode of 

 origin. This is undoubtedly different in the teeth of the two 

 jaws, as has been emphasized. But this latter consideration 

 may be regarded perhaps as suggesting comparisons between 

 the teeth of the two jaws which have not yet been closely 

 examined. On the views just advanced the one outgrowth of 

 the dental lamina beside the outgrowth which results in the 

 tooth of the adult is a residual lamina. Its form, moreover, 

 is highly suggestive of such an interpretation as is to be seen 

 in Text-figs. 1 and 4 ; and I have put forward other facts. 

 On the other hand, in the more mature teeth of the lower jaw 

 the shape of the whole tooth germ is not unhke that of the 

 Beluga as figured by Kiikenthal, as pointed out on another 

 page, and is of course also like that of the upper jaw of the 

 present species and specimen represented in Text-fig. 10 of the 

 present paper. Are there, in fact, after all, reasons for regarding 

 the process which ,1 have lettered ' c ' in the teeth of the lower 

 jaw (Text-fig. 4) as really the equivalent of the process lettered 

 'a' in the teeth of the upper jaw (e.g. Text-figs. 9, 10) '? 



If this is so, it is clear that a different view may have to be 

 taken of the homologies of the two teeth rudiments than that 

 advanced in the present paper. 



For if the labial outgrowth immediately following the lingual 

 outgrowth is a tooth germ, and not a residual lamina, it would 

 appear to follow that it is this which is the rudiment of the 

 tooth of the permanent dentition ; and therefore that the tooth 



