624 



the dental lamina, the latter structure taking on the function of the 

 former in this case. The condition present in the Mouse being merely 

 an exaggeration of that met with in the Rabbit, where the enamel 

 organs of the vestigial milk incisors are much reduced and to a great 

 extent fused with the necks of the corresponding structures belonging 

 to the large incisors, a very slight further reduction of those portions 

 of the enamel organs extending down the sides of the vestigial teeth 

 would produce the same condition as that present in the Mouse. 



We may therefore I think fairly conclude that this small den- 

 tinal cap present in the Mouse, represents the last stage in the re- 

 duction of a vanishing tooth the earlier stages of which are seen in 

 the Rabbit and Squirrel! 



That these vestgial incisors in the Rabbit, Squirrel and Mouse 

 represent the milk predecessor of the functional incisors I have not 

 the slightest doubt, and I think Freund is over cautious and inclined 

 to underestimate the facts which suggest that condition, when he 

 hesitates for a moment in deciding the homology of these tooth vestigies. 

 His doubt arises I imagine from the study of the palaeontological 

 evidence as adduced by Cope 10 ) for the ancestry of the Rodents and 

 a consequent desire to find a trace of the tooth which is seen to be 

 vanishing in such a form as Callamodon. If Freund had examined 

 frontal-sections in the region of the symphysis of the jaws instead 

 of only radial ones he would have seen that the small vestigial tooth 

 of the Rabbit was developed not only in front of, but also externo- 

 lateral to the point of attachment of the large incisor thus showing 

 conclusively that the former cannot represent the vestige of a tooth 

 situated nearer to the median line than is the large incisor, which is 

 the position of the vanishing tooth in the Tillodontia. 



Although I am desirous to prove that these minute teeth repre- 

 sent the milk predecessors of the large incisors and not a lost tooth 

 belonging to the same series as the latter, but situated more anteriorly, 

 yet at the same time I am fully in accord with Cope in the belief 

 that an incisor has dropped out between the front of the jaw and the 

 functional incisors of the adult. This I judge to have been the order 

 of suppression here from a comparison with the analogous case of the 

 incisors in the lower jaw of the Macropididae, in which group I have 

 shown ll ) that the single adult incisor is at least the 2nd tooth of the 

 series, as I found vestigies of teeth one on either side of it ; these vestigies 



10) E. D. Cope, On the mechanical Origin of the Teeth in the Ro- 

 dentia. Amer. Nat. XXII, p. 3 — 13. 



