625 



did not even attain the size of those of the rabbit, but nevertheless 

 they remained perfectly distinct and showed no trace of a fusion of 

 their enamel organs with that of the larger tooth. 



A comparison of this very similar condition of the incisors in 

 these two groups will I think cause one to doubt the suggestion of 

 Freund's that possibly were are dealing with the fusion of the germs 

 of two independent teeth, especially as this appears to be only un- 

 doubtedly met with in those animals whose jaw is crowded with teeth 

 which is certainly not the case in the Rabbit. 



These vestigial incisors in the Rodentia are developed from the 

 dental lamina immediately anterior and slightly lateral to the germ 

 of the functional teeth, each of the former is further developed from 

 that portion nearest to the surface of the gum whilst the functional 

 tooth is developed from the deeper part of the lamina, the minute 

 size of the former is due to the enormous dimensions and abnormal 

 early development of its successor which begins to cut the gum at 

 birth or just before. In short save for the fact that the enamel organ 

 of the small tooth is fused with that of its successor instead of being 

 connected by a variable length of neck as in the less specialized 

 teeth 12 ), the vestigial tooth has all the relations of a true milk tooth, 

 against which conclusion there can be no reasonable objection raised. 



The only other possible interpretation that might be offered, 

 would be to refer these tooth rudiments to the pre-milk dentition the 

 existence of which Leche 13 ) has supposed in order to explain the 

 minute teeth, found by him in Myrmecobius and with which the former 

 present a certain amount of resemblance in regard to position and 

 relative amount of development. 



Allowing him to be correct as to the existence of his supposed 

 earliest of all mammalian dentitions, I do not think it is probable 

 that we should find traces of such a vestigial structure persisting in 

 a specialized group like the Rodentia; the ancestry of which are to 

 be sought according to Cope in the generalized Tillodontia, who in 

 all probability possessed a typical milk dentition which has been 

 gradually suppressed as their descendants become more and more 

 specialized. 



11) M. F. Woodward, On the Development of the Teeth of the 

 Macropodidae. Proc. Zool. Soc, 1893, p. 450—473. 



12) This is the case in the vestigial teeth of the Squirrel according 

 to Fheuhd. 



13) "W. Leche, Nachträge zu „Studien üb. d. Entwickel. d. Zahnsystems 

 bei den Säugetieren." Morph. Jahrb., XX. Bd., 2. H., 1893, p. 113—142. 



