DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELES. 553 
enamel-organ, so-called (our de.), and the deciduous third pre- 
molar, dp. 3. 
The position of the latter with regard to the neighbouring 
dental lamina is seen to correspond—not to that of the other 
adult teeth—but most accurately with that of the canine 
** prelacteal ” enamel-organ, which is well seen in the section ; 
dp* and d? are both seen to project labially so as to lie ina 
sagittal plane distinct from that occupied by the developing 
enamel-germs of the permanent teeth. This relation is also 
very evident, if slightly less striking, in coronal sections 
through the same Anlagen ; we have only to imagine the bulk 
proportions of the respective parts in figs. 20 and 24 to be 
somewhat altered in order to realise the most complete corre- 
spondence between the two. 
It is of the greatest interest to compare Rése’s fig. 1, 
representing the dental condition of a foetal Phascolomys 
(11, p. 752) with that just described. There, in the wombat, 
it appears that dc. is comparatively large and calcified, but is 
evidently of precisely the same order as the smaller calcified 
cheek-tooth marked D, whose homology with dp. 3 cannot, in 
our opinion, be doubted (cf. also his fig. 3). Here, therefore, 
the bulk proportions of de. and dp. 38 are altered from what 
we find in Perameles, but otherwise the condition presents a 
remarkable similarity. 
The very marked labial situation occupied by dp3 in all the 
entire series of our stages is most striking, and must not: be 
confounded with such a lateral displacement of. enamel-organs 
as may ensue as a result of over-crowding of the jaw. This 
latter occurs more especially in the incisor region, and its 
effects are well illustrated in the figures of Rése’s models of 
Didelphys already referred to. But the distinction in the 
orientation of the enamel-organ of dp. 3 is established from 
its very first appearance. It is most striking when the germs 
of the anterior premolars begin to appear, and long before any 
displacement through encroachment on their part could be 
imagined. ‘Thus both in serial position and in period of 
differentiation the enamel-organ of dp. 3 relates itself to the 
