Mr. W. H. Flower on the Teeth in the Marsupialia. 181 
_ This change does not occur until the animal approaches the adult 
e. 
"5. Dasyuride.—In a foetal Thylacinus, in which no teeth had cut 
the gum, the crowns of the permanent incisors, canines, premolars, 
and anterior true molars were partially calcified, and necessarily 
much crowded together in the jaw. A very minute rudimentary 
molar was situated just beneath the alveolar mucous membrane, 
superficially to the apex of the hindermost premolar, and was evi- 
dently its milk-predecessor. 
6. Phascolomyide.—This family is placed last because the obser- 
vations regarding it are less complete than in the case of any of the 
others. The youngest Wombat available presented no evidence of 
succession of any of the teeth; but it is probable that the single 
premolar is preceded by a milk-molar, at a still earlier period than 
any examined. 
From the foregoing observations it may be concluded with toler- 
able safety that the animals of the Order Marsupialia present a pe- 
culiar condition of dental succession, uniform throughout the order, 
and distinct from that of all other mammals. This peculiarity may 
be thus briefly expressed. The teeth of Marsupials do not vertically 
displace and succeed other teeth, with the exception of a single tooth 
on each side of each jaw. The tooth in which a vertical succession 
takes place is always the corresponding or homologous tooth, being 
the hindermost of the premolar series*, which is preceded by a tooth 
having the characters, more or less strongly expressed, of a true 
molar. 
It has been usual to divide the class Mammalia, in regard to the 
mode of formation and succession of their teeth, into two groups—the 
Monophyodonts, or those that generate a single set of teeth, and the 
Diphyodonts, or those that generate two sets of teeth; but even in 
the most typical diphyodonts the successional process does not ex- 
tend to the whole of the teeth, always stopping short of those 
situated most posteriorly in each series. The Marsupials occupy 
an intermediate position, presenting as it were a rudimentary diphyo- 
dont condition, the successional process being confined to a single 
tooth on each side of each jaw. ‘This position, however, is by no 
means without analogy among the mammals of the placental series. 
In the Dugong and the existing Elephants the successional process 
is limited to the incisor teeth. It is questionable whether the first 
premolar of those animals of this group which have four premolar 
teeth, as the Hog, Dog (mandible), &c., ever has a deciduous pre- 
decessor, at all events so far advanced as to have reached the calci- 
fied stage. But the closest analogy with the marsupial mode of 
_ Succession is found among the Rodents. Here the incisors appear to 
have no deciduous predecessors ; and in the Beaver, Porcupine, and 
others, which have but four teeth of the molar series, 7. e. three true 
* The convenient distinction between false molars or premolars and true 
molars, is always well marked in the form of the crown, especially in the upper 
jaw, in the Marsupials. 
O* 
