GENERAL CHARACTERS 129 
attached for a considerable time, nourished by the milk injected 
into the mouth by compression of the muscle covering the 
mammary gland. ‘They are therefore the most typically mam- 
malian of the whole class. The nipples are nearly always concealed 
in a fold of the abdominal integument or “pouch” (marsupium) 
which serves to support and protect the young in their early 
helpless condition. 
Entering more fully into the characters of the subclass, which 
are also those of the order Marsupialia, it may be observed that the 
brain is generally small in proportion to the size of the animal, and 
the surface-folding of the cerebral hemispheres, though well marked 
in the larger species, is never very complex in character, and is 
absent in the medium-sized and smaller species. The arrangement 
of the folding of the inner wall of the cerebrum differs essentially 
from that of all known Eutheria, the hippocampal fissure being 
continued forward above the corpus callosum, which is of very 
small size. The anterior commissure is, on the other hand, greatly 
developed. 
The teeth are always divisible, according to their position and 
form, into incisors, canines, premolars, and molars; but they vary 
much in number and character in the different families. Except in 
the genus Phascolomys, the number of incisors in the upper and 
lower jaws is never equal. The true molars are very generally four 
in number on either side of each jaw. The chief peculiarity in the 
dentition lies, however, in the mode of succession. Thus there is no 
vertical displacement and succession of the teeth, except in the case 
of a single tooth on either side of each jaw, which is always the 
hindermost of the premolar series, and is preceded by a tooth 
having more or less of the characters of a true molar (see Fig. 34); 
this deciduous tooth 
being the only one 
comparable to the 
“milk-teeth ” of the 
diphyodont Eu- 
theria. In some 
cases (as in Poto- 
vous) this tooth re- 
tains its place and 
a = Fic. 34.—Teeth of upper jaw of Opossum (Didelphys mar- 
function until the supialis), all of which are unchanged, except the last premolar, 
animal has nearly, the place of which is occupied in the young animal by a molari- 
if not quite, attained form tooth, represented in the figure below the line of the other 
its full stature, and 
is not shed and replaced by its successor until after all the other 
teeth of the permanent series, including the posterior molars, are 
fully in place and use. In others, as the Thylacine, it is very 
rudimentary in form and size, being shed or absorbed before any 
