72(i 



that its Alliage is distieliical, i.e. that the Anlage has two rows, 

 while its final structure is harnast ichicai, i.e. that elements of both 

 rows t'ontribule to the construction. As to the distichical nature of 

 the inarsupialian dentition, this can of course be shown best wi;h 

 the dentition of polvprotodon'tic Marsupials. 



The fiu'ther discussion will lie based on the paitiaily de\eloped 

 dentition of a pouch-yonng of Peraiueles obesula, starting with the 

 lower jaw. In fig. 2 this Anlage has been sketched. In agreement 





Fig. 2. 



with other authors I found here a number of evolved but not further 

 developing small teeth. These have been dotted in fig. 2. 



The scheme in which the teeth are shown in their mutual topo- 

 graphical situation, represents the dental Anlage when the teeth-band 

 is viewed from the buccal side. It is evident that the tooth-germs 

 lie in two rows, one nearer the surface epithelium and one along 

 the lower margin of the teeth-band. There are eight teeth in the 

 exostichical row. numbered J — 8, the endostichical row also contains 

 eight teelh, indicated by a — h. The teelh of the two rows are clearlj' 

 seen to alternate. The object studied by me, from which fig. 2 has 

 been drawn, was at about the same stage of development of the 

 dentition as the object described as stage IV by Wilson and Hill 

 in their monograph on the evolution of the dentition of Perameles. 

 Of the eight exostichical teeth only three attain further development, 

 namely the third, sixth and seventh in the row, the other are reduced. 



The exostichical teeth are evolved from the lateral margin of the 

 teeth-band, as with the Reptiles. This is also the case with the 

 Monodelphian mammals, but still the Marsupials agree in this respect 

 much more with the Reptiles than with these mammals. How great 

 in young stages of development the agreement is between the Anlage 

 of the teeth of Marsupials and Reptiles, may be seen by comparing 

 figs. 3 and 4. In fig. 3 the teeth-band with the Anlage of an exostichical 

 and endostichical tooth of a young Lacerta is sketched, the exostichical 

 tooth lies parietally close below^ the epithelium of the cavity of the 

 mouth, the endostichical Anlage lies terminall}'. Fig. 4 is a repro- 

 duction of Wilson and Hill's fig. 37 and refers to the Anlage of 



