of the Tooth- Rudiments in Rodents. 325 



entire length of the diastema, but only in the upper jaw, which 

 was expressly denied by Pouchet and Chabry. 



Upon this dental fold, however, enamel-organs are never 

 developed in Lepus. After having lasted for a somewhat 

 lengthy period, it disappears without leaving a trace behind. 



In the upper jaw of Sciurus in the stage we examined a 

 dental fold was present, which was interrupted for a certain 

 distance behind the canals of Stenson, but was otherwise 

 greatly extended and continuous. According to Chabry and 

 Pouchet it is just in front of IStenson's canals that the fold is 

 wanting. Moreover it is asserted by the French authors, 

 though their statements upon this point are certainly some- 

 what confused, that in the lower jaw there is a complete 

 dental fold throughout the entire diastema. In our stage, 

 wliicli was but a little older, it was possible to distinguish 

 mere traces of a prolongation of the dental fold close behind 

 the rudiment of the incisor in the lower jaw, so that the state- 

 ment of MM. Chabry and Pouchet does not appear to me to 

 be a very credible one. On the other hand, we may with 

 confidence assume that in younger stages of the squirrel the 

 dental fold of the upper jaw will be found perfectly con- 

 tinuous in the diastema. 



In addition to this we find in Sciurus near the canals of 

 Stenson two enamel-organs lying one close behind the other, 

 of which the anterior is quite undeniably characterized as 

 such, while the posterior bears somewhat less distinct, but 

 nevertheless sufficiently definite, indications of its character. 

 It is remarkable that Chabry and Pouchet have figured one 

 of these rudiments, but, as we have seen, have interpreted it 

 quite incorrectly. 



How are these structures to be regarded? The anterior 

 enamel-organ can scarcely be regarded as a remnant of the 

 i. 2 which is present in Lejnis ; for in this animal the cord 

 connecting i, 2 with the e])ithelium of the oral cavity is 

 attached far in front of the orifices of Stenson's canals ; the 

 interval which is visible in fig. 14 (of the squirrel) between 

 this rudiment and that of i. 1 appears much too great for it 

 to be possible to assume that we are here dealing with two 

 tooth-rudiments following directly one after the other. It is 

 true that this question too will not be finally decided until we 

 have examined younger stages, since the possibility of a 

 secondary divergence of the two rudiments is also not excluded, 

 although it is not probable that this has taken place. More- 

 over it will be determined only by further investigations 

 whether a deposition of dentine still takes place in these 



