562 MR. M. F, WOODWARD ON [May 5, 
lamina; growing out from this labially is in both stages a small 
bud-like mass of cells (fig. 5, de.), in one case swollen at its free 
end, close to which is a small irregular calcification similar in 
appearance to the often vestigial de, but smaller in size: this is 
obviously the vanishing milk-tooth dec, the canine of the adult 
belonging to the replacing series. 
The Premolars. 
In the upper jaw there are three premolars, which Leche 
believes to be the 2nd, 3rd, and 4th respectively. It is true there 
is a slight gap between the canine and the anterior of these pre- 
molars, but there is a more conspicuous one between the two 
posterior teeth, and in this latter gap the dental lamina has a 
slight tendency to become specialized and enlarged (Plate XXIII. 
fig. 6b) ; but it is perhaps hardly large enough to be regarded as a 
tooth anlage, and, further, we know that when suppression affects 
the premolar series in the Placentalia, the Ist tooth of that series 
generally suffers suppression earliest. 
In the lower jaw there are only two premolars, but between 
them is a long stretch of dental lamina, which exhibits a most 
distinct development from its adamantine face; this growth is 
slightly indented by a specialized mass of mesoblast (fig. 7 6), the 
whole structure presenting a great similarity to a developing tooth : 
this, I believe, represents the last trace of a suppressed tooth, 
corresponding with the middle premolar above. 
The two posterior upper, and the posterior lower, premolars 
are present as functional teeth in both dentitions ; but the middle 
upper one is very variable, and is often wanting in some adult 
skulls, while in Z. micropus and £. pictus it is very minute. 
The anterior premolar, above and below, in both my stages, 
exhibits a large enamel-organ, bell-shaped in the older specimen ; 
attached to the dental lamina forming the necks of these struc- 
tures, on the labial side, is in each case a mass of epitheloid cells 
(figs. 8 and 8a, dpm.2); the free ends of these buds are swollen 
and flattened: closely applied to these is, in each case, an irregular 
calcification, resembling the most reduced stage ot de.; these are 
evidently reduced milk-premolars, the anterior functional premolars 
being then, as Leche supposed, replacing teeth. 
With regard to the last premolar, its milk representative 
resembles a molar in form, thus differing markedly from its 
successor ; a feature so characteristic of the 4th premolar of other 
Placentalia, that I think we may be quite safe in homologizing 
these two teeth with one another. 
If we examine the mutual relations of the 4th premolar and its 
successor during their development, we find that the replacing tooth, 
ppm.4, originates almost entirely in front of its supposed milk 
predecessor from the dental lamina between dpm.3 and dpm. 4, 
the enamel-organ of ppm.4 being more conspicuous in the sections 
in front and in the anterior region of dpm.4 than in its posterior 
