TOOTH-GERMS IX A WALLABY. 937 



alveolar epithelium, remaining as an isolated spherical mass. 

 It probably is the same structure described in JStage 1, and is 

 naturally of larger size. 



Lying at a deeper level but entii'ely independent of it, is the 

 anterior extrenjity of the large calcified functional incisor. That 

 these two structures have no morphological relationship with each 

 other is made clear, not only by their relative position but also 

 by the fact that the connection of the enamel organ with the 

 surface is definitely visible in a .section farther back, together 

 with a " concentric " vestige of a predecessor and an indication of 

 a potential successor. This large functional incisor we shall 

 indicate as cV , 



Then follows an interval of thirteen .slides without any trace of 

 teeth, and throughout the greater part of this interval the dental 

 lamina is scarcely visible. At length a definite " concentric 

 epithelial ina&s " occurs embedded in the dental lamina. It is im- 

 possible to say whether this is the vestige of a canine or of an outer 

 incisor. From its position relative to that of the opposing teeth, it 

 is probably the vestige of the latter tooth. The point in favour of 

 its being a vestigial canine is its propinquity to the first true post- 

 canine tooth. Howevei', it is a point of quite minor impoitance. 



The description already given of the true maxillary teeth 

 applies almost equally well to the corresponding sti'uctures in the 

 lower jaw. The only two points to which attention may be drawn 

 are (i) the lai^ger size and definite calcification of the 4th tooth of 

 the lower series; and (ii) the absence of a definite "concentric" 

 predecessor to the .same tooth, though there is a very minute 

 .structure which may indicate its remains, the position in relation 

 to the tooth itself being the correct one. That it should have 

 disappeared is wha,t would be expected from the greater size and 

 calcification of the successional tooth. 



III. HisroLOGV. 



Thei-e are certain histological peculiarities revealed by an 

 examination of these jaws. They are bi"iefly as follows : — 



(i) The oral epithelium along the alveolar margins is very thick 

 and heaped up to an unusual extent. In no other mammal that 

 we have examined have we ever seen this charactei' carried to such 

 excess. This is the " Zahnwall " of the German writers. It is 

 said to be a marked character in the Ungulates, and in our ex- 

 perience it is certainly not of common occurrence among mammalia 

 generally. Of course no morphological significance is implied 

 by the comparison. 



(ii) The somewhat unusual compactness of the stellate reticulum 

 and the definiteness of the stratum intermedium of the enamel 



(iii) The precocious development of the enamel which seems 

 to calcify as soon as the dentine, and even in some cases to 



