118 RODENTS. 
in a popular work like the present to point out its distinctive peculiarities without 
the aid of a series of figures. It may be mentioned, however, that the frontal 
region between the eye-sockets is much narrowed; and that in the zygomatic, or 
. cheek-arch, the cheek 
(jugal) bone is usually 
reduced to a mere 
‘splint occupying the 
middle of the arch. 
Again, the lower sur- 
face of the process 
arising from the upper 
jawhone to form the 
front half of the 
cheek-arch is more or 
less flattened out mto 
a nearly perpendicular 
plate; while the vacuity in the front of the eye for the passage of the nerves of 
the face is generally of considerable height, and wider above than below. 
The number of genera and species belonging to this family is so great that it 
is impossible to notice more than a small moiety in this work. Accordingly, only 
representatives of the various subfamilies into which these Rodents are divided, 
and such forms as are of general interest, are selected for description. 

SKELETON AND MOLARS OF BROWN RAT. 
THE AUSTRALIAN Rats. 
Genus Hydromys. 
The Australian rats, of which there is one aquatic species, together with an 
allied terrestrial form from the same region, constitute a subfamily distinguished 
from all the other mice by having only two pairs of molar teeth in each jaw ; these 
teeth being rooted, and having their crowns divided into distinct lobes. 
The Australian water-rat (Hydromys chrysogaster) is a rat-like aquatic Rodent, 
with broad and partially-webbed feet, the webs and claws being larger in the 
hinder than in the front-limbs. The fur of the back is black with an admixture 
of golden-coloured hairs; the under-parts being of a dark golden hue, save for a 
narrow stripe of flaxen running from the lower part of the neck to the middle of 
the belly. Except for its terminal fifth, which is flaxen-coloured, the tail is black. 
The tip of the muzzle is thickly covered with hairs, by means of which the nostrils 
can be closed. The length of the head and body is about 2 feet, and that of the 
tail half as much. The skull differs in the form of the front portion of the cheek- 
arch from the ordinary murine type. This species is confined to Australia and 
Tasmania, where it is known to the colonists as the beaver-rat. It is thoroughly 
aquatic in its habits, and is not uncommon on the banks of the rivers; while it is 
at times seen on the seashore. It is an inhabitant of Australia, Tasmania, and 
New Guinea. 
The Queensland rat (Xeronvys myoides) is a terrestrial species of about double 
