626 



There is however one slight point which might be considered as 

 supporting the above suggestion, viz : — there is present at one stage 

 in the development of the large lower incisors a slight downward 

 prolongation of the dental lamina on the posterior side of the enamel 

 organ of that tooth. This might be regarded as presenting an indi- 

 cation of a permanent tooth, the large incisor being referred to the 

 milk dentition, while the vestigial incisor in front might represent the 

 pre-milk dentition. On the other hand if this very slight prolonga- 

 tion of the dental lamina has any morphological value at all I should 

 be inclined rather to consider that it represented that supposed fourth 

 dentition as described by Kükenthal 14 ) and Leche 13 ) for the Seals. 



The Squirrel, the Rabbit and the Mouse present us with all the 

 stages in the suppression of the vestigial milk incisors, in the former 

 their relation to their successors is more typical, in the Rabbit where 

 the tooth of the second dentition is more largely developed, the milk 

 predecessor has partially lost its independent enamel organ and lastly 

 in the Mouse where the successional tooth in still more out of pro- 

 portion, the enamel organ of the milk tooth has completely dis- 

 appeared, the tooth itself being very variable and when present exces- 

 sively minute. 



Some confusion seems still to exist as to the age at which the 

 Rabbit looses its milk teeth, so I give here a short summary of the 

 facts as I have observed them. 



Milk incisors No. 1. £ never cut the gum, absorbed in utero. 

 Milk incisors No. 2. £ functional, shed at about the third week 

 of extra uterine life, not shortly before or 

 after birth as Freund states. 

 Milk premolars | functional, shed at about three weeks, 



with the last. 



With regard to the two tooth vestigies described by Freund in 

 the Squirrel as occurring behind the 1st upper incisor, he suggests 

 that they may represent an incisor and a canine, and he especially 

 lays stress on the position of the former along side of Stenson's 

 canal to prove that it is not the 2nd incisor of the Rabbit. Now 

 in most other Mammals that I am familiar with the 2nd functional 

 incisor is invariably developed beside that canal so that its position 

 in that Rabbit is obviously a specialized one as it is situated imme- 



14) W. KüKENTBAL, Entwickelungsgesch. Untersuch, am Pinnipedier- 

 gebisse. Jen. Zeitschr. f. Naturwiss., Bd. XXVIII, N. F. XXI, 1894, 

 p. 76 — 118. 



