1-54 J. T. WILSON AND J. P. HILL. 



cusp of "y," tlie anterior end of the enamel-organ of " y " 

 overlapping the hinder end of " x " on its mesial aspect. 



In other respects the character and position of the nodules 

 in the lower jaw require no further illustration than that 

 which is afforded by text-fig. 2. 



That the above arrangements are not merely fortuitous is 

 further testified to by the examination of the section-series 

 from the opposite side of the head of the same specimen. The 

 condition there existing is practically identical with that set 

 forth in text-fig. 2. The only noteworthy deviations from 

 this scheme are : {a) that the well- calcified vestigial toothlet 

 in the upper jaw^ illustrated in figs. 9 and 11 (and which we 

 have indicated by the symbol " dy~ " in the scheme in text- 

 fig. 2), is on tliis side represented by a concentric epithelial 

 nodule of precisely similar character to the other nodules 

 with which it is in series^, and (b) that the anterior of the 

 two nodules {" dy^ "), which in text-fig. 2 are seen to lie 

 opposite the adjacent ends of the two larg'e lower enamel- 

 organs '' X " and " y," is absent on this side. It has already 

 been stated that it is comparatively small and insignificant on 

 the other side, so that its absence on the side now under con- 

 sideration is the less surprising. 



(3) The morphological character of the nodules. — 

 From the foregoing account it will already be apparent that 

 we regard the series of epithelial nodules as constituting a 

 series of vestigial representatives of an earlier tooth genera- 

 tion. That they must be so regarded has already been 

 surmised by Marett-Tims (7) on the basis of Poulton's 

 original account of them. Tims was apparently led to this 

 interpretation of the structure in question through his own 

 observations (6) of the presence of corresponding structures 

 in connection with the dentition of other mammals, structures 

 which seemed to him to demand a like explanation. 



The evidences in favour of such an interpretation may be 

 summarised as follows : 



(a) The nodules are not fortuitously distributed, but form a 

 more or less regular series both as regards number and posi- 



