Rodentia. MAMMALIA. Rodektia. 1 29 
number from fifty to one hundred or more ; neverthe- 
less their habits are strictly monogamous. Before 
going to sleep on the floating ice-fields they take the 
precaution of appointing sentinels, who, when any 
danger threatens, forthwith rouse the entire troop, by 
uttering loud bellowing cries, and instantly all are seen 
tumbling over into the sea, where they are tolerably 
secure. If any are wounded the remainder display 
much sympathy, the mothers defending and carrying 
oft' their young with the greatest ardour. On several 
occasions they have been known to attack a boat’s 
crew, as happened, for example, to two officers who 
went walrus-hunting by themselves, near Waggat’s 
Straits, in the year 1773. They had succeeded in 
wounding a solitary one, which immediately dived 
under water, and after a short time returned with 
several others, attacked the officers, wrested from 
them an oar, and very nearly succeeded in capsizing 
the boat; fortunately another boat came to their assist- 
ance, and the infuriated animals were driven off. At 
all times the capture of the Walrus is attended with 
considerable difficulty, for, although not naturally a 
shy animal, it has learned to dread its human adver- 
sary, and therefore takes to the water on the slightest 
alarm ; moreover, the skin is sufficiently hard to resist 
the harpoon, unless it has been driven home with great 
force. The tusks are much prized; their ivory being, 
bulk for bulk, far more valuable than that of the 
elephant; the texture of the dentine is denser, and the 
colour of a purer white. The AValrus is omnivorous, 
and in its stomach there have been found remains of 
young seals, fish, shrimps, and other Crustacea ; also 
various kinds of molluscs and sea- weeds, especially of 
the common kind — Fiicus digitatus. 
Those who desire further information respecting the 
habits and mode of capturing the Walrus, we beg 
especially to refer to the thirteenth and thirtieth 
chapters of the first volume of Dr. Kane’s “ Arctic 
Explorations,” where they will find most interesting 
details, for which we have here no space. 
Order VII.— RODENTIA. 
The Rodents constitute a well-defined natural group, 
comprehending a great multitude of comparatively small 
species, all of which are characterized by the possession 
of peculiar incisor teeth. These organs are usually 
four in number, two occupying the upper, and two the 
lower jaw ; they are also placed prominently forward 
below the muzzle, and are separated from the molar 
teeth by a considerable mterspace (fig. 41). Their 
office is essentially that of gnawing ; hence the . Rodents 
are sometimes called gnawers, or rongeurs by the 
French. The form of each incisor tooth resembles a 
chisel, the anterior and superior edge being remarkably 
sharp and trenchant; the tooth is like^vise so con- 
structed that its tissue, and therefore function also, 
is in no way damaged by continuous use ; on the 
contrary, every time it is put in action, the weapon 
chisels down the hard substances required for food or 
other purposes, while, at the same time, it sharpens 
Fig. 41. 
quickly than the enamel, which is consequently left 
standing prominently forward in the form of a chisel- 
like process. The molar teeth are few in uiunber, 
Fig. 42. 
Molai' Teeth of tlie Beaver. 
seldom exceeding four on each side of either jaw ; they 
are flat, and have the enamel arranged in the form of 
transverse plates, which, during the backward and for- 
ward movements of the jaw, act antagonistically with 
the corresponding ridges of the opposed teeth (fig. 42). 
In order to insure and facilitate this antero-posterior 
movement, and the converse action, the condjde of the 
lower jaw is articulated to the skull by a longitudinally- 
formed socket, which admits of scarcely any lateral 
motion. There are no canine teeth in the Rodentia ; 
and from the several dental peculiarities here men- 
tioned, it may readily be perceived that the food of these 
animals must be chiefly of a frugivorous nature, and that 
it will comprise substances of the hardest character, 
such as roots, the bark of trees, and even wood itself. 
Some of the species, however, are omnivorous, feeding 
on other animals, as well as on various vegetable mat- 
ters ; and in these we find the molar teeth more or less 
tuberculated. The alimentary canal is of great length, 
the coecum being often remarkably large, in some cases 
exceeding the stomach in size, and filling up the larger 
portion of the abdominal cavity. A curious exception 
j is seen in the dormouse, where the ccecum is entirely 
i wanting. The form of the stomach in Rodents is sim- 
Skull of the Rabbit. 
itself, and is thus always fit for use. This interesting 
result depends, for its integrity, on the following struc- 
tural arrangement : The anterior and convex surface of 
the organ is coated with a thin layer of hard enamel, 
the central mass consisting of the somewhat less dense, 
but still tolerably strong ivory ; and from this disposi- 
tion of the two structures, it will readily be perceived 
that, during attrition, the ivory must wear away more 
Yon. I. 17 
