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- 

 rous) this tooth re- 

 tains its place and 



f t until fhp Fl0 ' 34 - Teeth of upper jaw of Opossum (Didelphys mar- 



J 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 f rm 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 



9 



