446 
MR. O. THOMAS OR THE HOMOLOGIES AND 
As to the milk dentition, those species that have a large permanent pm 4 have 
a distinct tricuspid milk-tooth preceding it (Plate 27, fig. 6), and persisting until 
a comparatively late period of life.* On the other hand, when the permanent pm 4 
shows a, tendency to disappear, the milk-tooth would seem to he first gradually 
aborted ; thus in Ph. donee , where the permanent tooth is of a medium size, the milk- 
tooth is quite minute and functionless, while in the still smaller-toothed Ph. apiccdis , 
and also in Ph. wcdlacei, I have been unable to find any trace of a milk-tooth in the 
only young specimens available—the permanent tooth, however, as in the other species, 
still rising into its place considerably later than any of its neighbours. The actual 
calcification of this tooth seems also to take place much later in PJiascologale than in 
any other of the tooth-changing Marsupials, so that the tooth is often not to be found 
beneath the bone until a very short time before its eruption. 
From these observations it is clear that the normal state of a member of the present 
group is to have three well-developed premolars, the last one of which has a milk 
predecessor. Then a tooth-reduction has taken place, all of which has fallen on what 
is evidently a peculiarly plastic tooth, viz., pin 4 , and this, with the milk-tooth 
preceding it, has been decreased in various degrees, and in the end altogether 
suppressed, as in the allied genera Dasyurus (Plate 27, fig. 5) and Sarcophilus. 
Having thus found out which of the three premolars present in Phascologale has 
disappeared in Dasyurus and Sarcopliilus, we have, before we can settle the proper 
homologies of even these three, to discover which of the full number of four premolars, 
still possessed by the Eutheria, has disappeared in Phascologale and other Marsupials, 
for it has always been the natural presumption that four, and not only three, was the 
original typical number of premolars as much among the Marsupial as among the 
Placental Mammals.t Since no species now living, however, shows this number, that 
presumption has hitherto remained unproved, and still less has it been proved which 
one of the full set of four has disappeared to leave the common number of three, most 
authors jumping to the conclusion that, as in so many Carnivora, it is pm 1 that has 
been suppressed. Now 7 , however, I am at last able to prove the first, and make out 
the second point to my ow r n satisfaction, and to that of both Professor Flcdyer and 
Mr. Lydekker. 
When looking at a somewhat abnormal skull of Dasyurus maculatus, I was struck 
by seeing a minute projection attached to the gum, between the two premolars, and, 
being on the look-out for such a thing, I immediately suspected that it might be the 
* The specimen of Ph.penicillata Avitli milk dentition, from which the figure is drawn,has its third molar 
np and in place, and has a basal length of nearly 40 mm. as compared to about 45 mm. in fully adult 
specimens. 
f Although some even of the highest authorities look upon three as the typical or parent number of 
premolars in the Marsupials ; see, for example, Tomes’ ‘ Dental Anatomy ’ (2nd ed.), 1882, p. 420, where, 
apart from this point, a most excellent account is given of the structure and development of Marsupial 
teeth. 
