826 On the Develojwient of the Tooth-Rudiments in Rodents, 



rudiments or not, and liow they disappear. Taking the 

 conditions as we find them in the stage under consideration, 

 we might regard these two enamel-organs as remnants of 

 rudiments of a posterior incisor and a canine tooth wliich 

 have been strangely preserved; their interpretation as remnants 

 of premolars is opposed by tlieir position beside the orifices 

 of Stenson's canals. 



The reader may here once more be reminded that in Gavia 

 only very doubtful remnants of the dental fold were distin- 

 guishable in the diastema, while in Cricetus and Mus no trace 

 of a rudimentary embryonic dental germ was visible. 



In conclusion, it may be also pointed out that our results 

 correspond to Fleischmann's view, according to which the 

 degeneration of the tooth-rudiments has advanced further 

 in the lower than in the upper jaw. Apart from the doubtful 

 statements of MM. Chabry and Pouchet with regard to the 

 squirrel, no dental fold was found in the diastema in the 

 lower jaw ; in the upper jaw, on the other hand, a structure 

 of this kind was distinguishable in a condition of greater or 

 less completeness in Lepus^ Cavia, and Sciurus, while in the 

 case of the last-mentioned form it is even provided with 

 enamel-organs. It also agrees very well with the general 

 views of Schlosser and Fleischmann, that the Lagomorpha 

 represent an especially old and conservative Rodent type, and 

 that after these the Sciuromorpha come next in order ; that in 

 representatives of these groups it was possible to demonstrate 

 the most distinct and most widely extended embryonic remains 

 of tooth-rudiments which have disappeared ; while in the 

 more highly modified Myomorpha, in so far as these were 

 investigated, the embryonic reduction was also more com- 

 plete. Lastly, stress may likewise be laid upon the fact that 

 the discovery of a dental fold in the diastema in the case of 

 Lepus and Sciurus is in conformity with the similar disco- 

 veries of other authors in the case of different Mammals with 

 incomplete dentitions. Thus the presence of a dental fold in 

 the diastema in the premaxilla of Ruminantia was not long- 

 since finally determined by Mayo *. For the Edentata the 

 same was proved by Chabry and Pouchet, and it is well 

 known that in the case of the whalebone whales Geoffroy 

 St.-Hilaire has demonstrated the existence not only of the 

 dental fold, but even of entire embryonic teeth, which merely 

 fail to cut the gum and are absorbed within the jaw. 



* Mayo, " The Superior Incisors and Canine Teeth of Sheep " (two 

 plates), ' Bulletin of the Museum of Comparative Zoology at Harvard 

 College,' vol. viii. (Cambridge, 1886-1«88). 



