731 



incisors of a Monodelphian mammal, the three that develop with 

 the three incisors of the permanent dentition. In this way a natural 

 connection is established between the number of incisors of the two 

 groups. Here I shall restrict myself to a simple mentioning of this 

 view, remarking in passing that e.g. of the tive incisors developing 

 with Didelphis three will belong to one series, two to the other. 

 So the haraastichical character of the dentition is still very strongly 

 expressed there. 



It has been shown above that also the series of incisors in the 

 lower jaw of Perameles is hamastichical, the first and second incisor 

 being endostichicai, the third exostichical. For ihe upper jaw this 

 is not so easily proved, since tliere the incisors are evolved more 

 directly and individually from the buccal epithelium. Five incisors 

 develop here in all. As many are evolved in the lower jaw and 

 phenomena, observed in older specimens, rendered it probable that 

 also these tive would be evolved in two rows. To prove this more 

 fully would take up too much space here. That the tive incisors 

 in the upper jaw correspond with the five, evolved in the lower 

 jaw, follows still from the fact that rudimentary incisors do not 

 occur in the upper jaw. 



I need not enlarge here on Ihe e.xostichical character of tooth 6. 

 From this tooth-germ the premolar is formed whicii undergoes sub- 

 slitulioii Now in the domain of tooih 6 and 7 an iri'egularity in 

 the constitution of the dentition is met with. As far as tooth 6 the 

 elements of the two series alternate regularly and so after the exosti- 

 chical tooth ti an endostichicai one would have to follow. But this 

 tooth is still lacking at the stage of lig. 2. Still at the spot marked 

 by a cross the free border of the dental lamina is already slightly 

 thickened and with further development the endostichicai tooth will 

 here be formed which is destined to replace the exostichical tuolh 

 3 (the third premolar). Wilson and Hii.l have described these stages 

 more fully. It should be particularly pointed out here that this sub- 

 stituting tooth is formed in the same way from the fVee bolder of 

 the dental lamina as the teeth (/, b, c etc., for these also are formed 

 terminally. So the substituting tooth is isostichical with the teetli 

 a, b etc., the strong development of tooth six however seems to be 

 the cause of its later Anlage. 



After tooth 6 now follows tooth 7 and concerning this one I 

 disagree with Wilson and Hill in this respect that according to 

 my view also this dental Anlage, from which the first molar will 

 develop, belongs to the exostichos. If this view is right, this would 

 mean that with Perameles also the first molar belongs to the exo- 



