SUCCESSION OE THE TEETH IN THE MAESUPIALIA. 
635 
A comparison of the dentition of a specimen at this age with that of an adult animal, 
will be quite sufficient to enable us to fill in the intermediate stages. The change 
which takes place is only the substitution of the reserve premolar for the temporary 
molar, and the complete evolution of the other teeth, including often a second rudi- 
mental premolar*. 
Phascolarctos. — The skull of the youngest Koala that I have been able to examine is 
3 inches in length, that of the adult being 5. In this the permanent incisors and canines 
are in place, though not quite so prominent as in the adult. The single permanent pre- 
molar, and the first and second true molars are also in place, and the germs of the 
remaining two molars are calcified. We may infer from this, that if the premolar 
replaces a temporary molar, which according to analogy with the allied genus Pha- 
langista, and, as will be shown, with all other marsupials the succession of whose teeth 
is known, is most probable, this must take place at a very early age. 
Family Peramelida:. 
I have not had an opportunity of observing the earlier stages of dentition in any mem- 
ber of this family, but when the animal is not quite full grown, the teeth are in the con- 
dition shown in Plate XXX. fig. 1. The permanent incisors, canines, and two anterior 
premolars are in place. Behind these in each jaw is a very minute, rather compressed 
tuberculated tooth, succeeded posteriorly by the true molars of the permanent series, 
In the alveolus above this minute tooth, which is the temporary or deciduous molar, is 
lodged the germ of the posterior permanent premolar, a tooth having a large compressed, 
pointed triangular crown, with small anterior and posterior basal tubercles, seen in situ 
in the figure of the adult dentition, Plate XXX. fig. 2, coloured blue. 
In an immature specimen of Cheer opus the dentition is exactly in the same stage as 
shown in the Perameles (Plate XXX. fig. 1), but the temporary molar is of still smaller 
relative dimensions. 
Family Didelphida:. 
In a young Opossum of one of the larger species (probably Didelphys virginiana), 
which measured 7-5 inches long, the tail being 2*25 and the head 175, the extreme 
points of the incisors, canines, premolars, and first molars had just pierced the gum. The 
state of development of these teeth is shown in Plate XXX. fig. 3 ; the incisors, canines, 
and first two premolars corresponded exactly with those of the adult animal. The suc- 
ceeding tooth above and below, which in the adult is a compressed triangular premolar, 
was here a low-crowned, broad tooth, bristling with cusps like the true molars. Beneath 
it, in the lower jaw only, a minute yellow capsule of a successional tooth was found. 
In a half-grown Virginian Opossum the incisors, canines, and first two premolars were 
fully developed, but more crowded together in the jaws than in the adult (see Plate 
* I have confirmed these observations by the examination of a large series of skulls of Phalangers of 
various ages in the Leyden Museum. 
