RODENTIA. MAMMALIA. RODENTIA. 



129 



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 

 off 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 Walrus 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 Fucus 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 interspace (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 likewise 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. 



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 

 VOL. I. 



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 number, 



Molar Teeth of the Beaver. 



seldom exceeding four on each side of either jaw ; they 

 are flat, arid 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 condyle 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 

 is seen in the dormouse, where the coecum is entirely 

 wanting. The form of the stomach in Rodents is sim- 



