SUCCESSION OF THE TEETH IN THE DASYURID2E. 
447 
missing premolar, present through atavism. I then turned to Phascologale, in which, 
as pm 4 is still retained, there seemed more hope of tracing the other missing premolar ; 
but here, in all the commoner species, the teeth fit so closely against one another that 
a functionless atavistic tooth would have no chance of developing. In certain of the 
rarer Papuan species, however, the teeth, owing to the length of the muzzle, are more 
separated from one another, and, on examining all the available examples of these, 
I was rewarded by finding on one side of the upper jaw, in a specimen of Ph. dorsalis 
belonging to the Genoa Museum,* a large and distinct tooth(Plate 27, fig. 7) protruding 
from the gum between the teeth corresponding to the two anterior premolars in an 
ordinary Phascologale ,t and in exactly the same position, therefore, as the minute 
rudiments previously found in Dasyurus. The tooth itself is two-rooted, precisely 
similar in shape to the other premolars, and is of about half the size of the first 
premolar in the same species. This, then, was clearly the missing premolar, and that 
here was its most natural place is shown by the extreme frequency with which a 
marked and prominent gap exists at this point in adult Marsupials, as, for example, 
in Didelphys, Perameles, and others. 
That the original typical number of premolars in the Mammalia was four is also 
strikingly exemplified by several of the earlier fossil Marsupials—as, for example, by 
Triconodon, which has the full cheek-teeth formula of four premolars and four molars ; 
by Ctenacodon, Plagiaulax minor, and others ; thus proving beyond question that 
the pm 3 discovered in the recent Phascologale, is really an atavism and not a mere 
meaningless abnormality. 
It results from this discovery as to the position of the missing premolar that in all 
the numerous PolyprotodontJ Marsupials with three premolars these are homologous 
with the first, third, and fourth of the normal Mammalian dentition, and not with the 
second, third, and fourth, as has ordinarily been presumed to be the case. 
For the abnormal specimen of Phascologale dorsalis we therefore obtain (on one side 
only) the following premolar formula :—P.M. 
1 . 2 . 3 . 4 § 
1 . 0 . 3.4 
, from which the suppression 
of the upper pm 3 gives us P.M. 
1 . 0 . 3.4 
1 . 0 . 3.4 
, the formula in Tliylacinus, Phascologale, 
* For the loan of which, and of a large series of other Papuan specimens, I have to thank my friend, 
the Marquis G. Doria, Director of that Museum. 
t Compare the teeth of the other side in the same specimen (Plate 27, fig. 8, l'eversed) for the coiTes- 
pondence between the different premolai'S. 
X As to the Diprotodonta with three premolars, although, on the one hand, in the Mesozoic Plagi- 
aulacidee it was clearly pm 1 that was first lost, yet, on the other, the positions of the premolars in certain 
of the modern PhalangistidEe are such as to raise a suspicion that they also, like the Polypi’otodonts, have 
lost pm 3 rather than pm 1 ; but in any case the loss has been independent of that in the Polyprotodonts, 
both groups having' had four premolai’s some time after their sepai’ation from one another. 
§ I have found this method of writing dental foi’mulae far superior to the ordinai’y one, as by it, not 
only the number, but the homologies of the teeth are clearly shown. Each tooth has its sei-ial number, 
which is written in if the tooth is pi'esent, but is l’eplaced by a cipher if not. 
