582 ME. M. F. woodwaud om [May 5, 



more constant in connection with the 1st molars, yet were also 

 found in the region of the 2nd molars in several genera. 



The lingual continuation of the dental lamina was found in all 

 six genera examined, whereas the labial growth was more irregular 

 and only observed in three forms ; this latter growth was most 

 conspicuous in Erinaceus, where it was constant from the beginning 

 of m. 1 to the end of m- 2 . 



A great deal of stress has been laid upon the presence of these 

 structures, especially that of the lingual one, its presence having 

 been said to prove that the molar teeth belonged to the milk- 

 dentition. There is no doubt that if we simply compare such a section 

 as fig. 26 (Plate XXVI.), representing the molar tooth of Centetes, 

 with a developing milk-tooth which is known to have a successor, 

 we should certainly conclude that the lingual growth of the dental 

 lamina in the two cases was the same structure ; and as it can in 

 one case be shown to give rise to the enamel-organ of a replacing 

 tooth, we might apparently be justified in concluding that in the 

 case of the molar it represented a reduced enamel-germ of a 

 permanent tooth, and that the molar tooth belonged in consequence 

 to the milk-dentition. But it is now well known that we have 

 in the Mammalia traces of three or four sets of teeth ; and as it is 

 highly probable that the Mammalia are derived from polyphyodont 

 ancestors, it is possible that (here might at any time appear traces 

 of a polyphyodont dentition. It appears, then, to me that presence 

 of a lingual continuation of the dental lamina does not necessarily 

 imply that the labial tooth belongs to the milk-series ; it might 

 equally well belong to the permanent or to the post-permanent series, 

 all traces of the earlier labial sets being lost, the lingual growth 

 being not merely the enamel-germ of a successor, but the free end 

 of the undifierentiated dental lamina, which may go on growing 

 and producing fresh sets of teeth, as it does in the polyphyodont 

 reptile, where it is the " anlage" of numerous enamel-organs. 



Compare for a moment these two diagrams (p. 583) : fig. 1 repre- 

 senting a section of the dental lamina of a reptile with a practically 

 unlimited succession, while fig. 2 represents the milk-tooth of a 

 mammal with a lingual development of the dental lamina, which is 

 here known to give rise to a permanent tooth ; we should not in 

 this case be justified in concluding that " m " in fig. 2 was the 

 homologue of 3 in fig. 1, merely because of the presence of this 

 similar development of the dental lamina on its lingual side ; we 

 must either conclude that m is the homologue of 1 and the 

 permanent tooth of 2, or perhaps 1 or both 1 and 2 have been 

 completely suppressed, and therefore m is the homologue of 2 or 3 

 as the case may be. In fact we must start with the dental lamina 

 from the gum, looking most carefully for labial rudiments, so as to 

 be perfectly sure that none of the earlier sets of teeth have 

 disappeared, before we can homologize the functional teeth, and we 

 must naturally expect to find a lingual growth of the dental lamina 

 constantly present, whether we are dealing with the 1st, 2iid, or 

 3rd sets, there being no reason to believe that there is an ultimate 



