36 
SYSTEM OF NATURE. 
that "whereas the whales among placentals lead imme¬ 
diately to the fishes, while the marsupials occupy exactly 
the opposite, and indeed the preeminently terrestrial 
part of the vertebrate group, these latter cannot, on 
the principle of approximating like to like, possess a 
swimming group equal to that of the whales among the 
placentals. 
PLAC ENT ALIA. MARSUPIALIA. 
SCANDENTIA 
Lemur 
Didelphys 
Rapacia 
Hyaena 
= Thylacinus 
VOLITANTIA 
V espertilio 
= Pterodactylus ? 
Saltantia 
Pteromys 
= Petaurus * 
Repentia 
Manis 
= Echidna 
Ambulantia 
Auchenia 
= Macropus 
Natantia 
Manatus 
= Ornithorhynchus. 
The principal groups of marsupials at present known 
are indicated by the genera above named. Dasyurus of 
course classes with Thylacinus, Phalangista with Petaurus, 
Hypsiprymnus with Macropus, &c. 
The number of ascertained existing marsupials com¬ 
pared with that of the placentals is perhaps small, and 
the series therefore imperfect, but we must recollect that 
this number is daily increasing. Mr. Waterhouse’s vo¬ 
lume contains descriptions of 105 species, and many 
others have been added, although little more than a 
year has elapsed since its publication. The remains of 
extinct marsupials appear to be very abundant: many 
species and a few distinct genera have been ascertained. 
And here I would indulge in a little hypothesis, and sug- 
* The wombat appears to be a direct approach to the Glires. 
