vil THE PLACENTA B25 
a special sucking mouth. This sucking mouth is an extra-uterine 
production, and is of course an adaptation to the particular needs 
of the young, just as are other larval organs, such as the chin- 
suckers of the tadpole, or the regular ciliated bands of the 
larvae of various marine invertebrate organisms. 
There are a number of other features which distinguish the 
Marsupials from other mammals. 
The cloaca of the Marsupials is somewhat reduced, but is still 
recognisable. Its margins in TVarsipes are even raised into a 
wall, which projects from the body. 
The tooth series of the Marsupials was once held to consist of 
one dentition only, with the exception of the last premolar, which 
has a forerunner. The interpretation of the teeth of Marsupials 
are various. Perhaps most authorities regard the teeth as being 
of the milk dentition, with the exception of course of the single 
tooth that has an obvious forerunner. But there are some who 
hold that the teeth are of the permanent dentition. In any case 
it is proved that a set of rudimentary teeth are developed before 
those which persist. Those who believe in the persisting milk 
dentition describe these as prelacteal. Another matter of in- 
portance about the teeth of this order of mammals is that their 
numbers are sometimes in excess of the typical Eutherian 44. 
This, however, holds good of the Polyprotodonts only. 
It was for a long time held that the Marsupials differed 
from all other mammals in having no allantoic placenta. But 
quite recently this supposed difference has been proved to be 
not universal by the discovery in Perameles of a true allantoic 
placenta. The Marsupials have been sometimes called the Di- 
delphia. This is on account of the fact that the uterus and the 
vagina are double. Very frequently the two uteri fuse above, 
and from the point of junction an unpaired descending passage 
is formed (see Fig. 48 on p. 74). | 
A character of the brain of Marsupials has been the subject 
of some controversy. Sir Richard Owen stated many years ago 
that they were to be distinguished from the higher mammals by 
the absence of the corpus callosum. Later still it was urged 
that a true corpus callosum, though a small one, was present ; 
while, finally, Professor Symington! seems to have shown that 
1 “The Cerebral Commissures in the Marsupialia and Monotremata,” Journ. 
Anat. Phys. xxvii. 1893, p. 69. 
