580 MR, M. F, WOODWARD ON [May 5, 
tooth; at the horns of the crescents, 1. ¢. at the anterior and 
posterior extremities of the outer border, and in the middle of 
this edge where the two crescents meet, slight additional cones are 
raised up ; these, however, appear very late (Plate XX VI. fig. 35, 
a & b, 1, 2, 3, 4). 
Though no less than four stages were examined, yet it was not 
quite possible to determine which cusp was the first to appear, 
for even in the earliest stage of m. 3 two slight prominences were 
already visible corresponding to the paracone and metacone. In 
the case of m.1 and m.2, three cusps were present in all stages, 
but of these the two external were alone conspicuous in the 
younger stages, the antero-external (paracone) being the largest, 
though in the adult it is smaller than the metacone; this, I think, 
shows that the paracone is the first to develop. The internal pro- 
tocone (7) appears late as a low inward extension from the base of 
the paracone (fig. 32) and cannot possibly be regarded as the original 
axis of the tooth. The 4th cusp to appear is the small anterior 
external cusp, which is connected with the anterior slope of the 
paracone, the hypocone evidently appearing very late. 
In the lower molars the protoconid forms the main axis of the 
dentine germ, and develops long before any of the other cusps, the 
next in order being the metaconid, followed by the hypoconid and 
entoconid, and lastly the paraconid. The heel itself minus its two 
cusps is developed very early before even the metaconid. The 
paraconid is especially late in its development; consequently 
the molar tooth before this cusp appears presents a very curious 
shape, the entire antero-external region of the dentine germ beiug 
absent. 
Upper molars. Lower molars. 
1. Paracone. 1. Protoconid. 
2. Metacone. 2, Metaconid. 
3. Protocone. 3. Hypoconid. 
4. Small antero-external. 4. Entoconid. 
5. Hypocone. 5. Paraconid. 
GENERAL COMPARISION OF RESULTS. 
The 4th Premolar. 
The homology of the 4th premolar of the Placentalia with the 
posterior premolar of the Marsupialia was first pointed out by 
Thomas, and there can be no doubt that this tooth in the two 
groups presents certain constant and striking features ; thus dpm. 4 
is nearly always molariform, whereas ppm. 4 is often almost unique 
in its pattern, being a highly specialized tooth, which in those 
cases where it resembles any other tooth has a striking similarity 
to dpm. 3 (Hypsiprymnus, Canis, &c.). 
Some time ago I pointed out that in Macropus the so-called 
ppm. 4 developed from the dental lamina between dpm. 3 and 
dpm. 4 (28, pl. 36, fig. 19), and was evidently serially homologous 
