sharp, and are evidently adapted for seizing and holding their prey, and afterwards tearing in pieces the flesh and other soft parts of the animals. The conical and very slightly-curved form of these teeth, united with their sharpness and strength, is the best that can be imagined for effecting this object. The

cheek-teeth have for the most part only cutting edges, and those of the lower jaw shut within the upper, passing them so closely as to form an accurate instrument either for shearing off pieces from the flesh, or for subdividing the portions which have been torn by the canine teeth. On each of them are sharp triangular processes, which greatly facilitate the entrance of the tooth into the flesh. The range of these teeth is short, as is also the whole jaw, by which great power is gained in this particular direction. The articulation of the lower jaw is also circumscribed to a perpendicular motion, the only one which the structure of the teeth would permit; and the masticatory muscles are of enormous size and strength, particularly in the hyæna, to enable these ignoble violators of the grave to crush the bone and cartilages which form a considerable part of their bill-of-fare.

The animals of the bear tribe on the other hand, whose diet is chiefly of a vegetable nature, have an elongated jaw, canine teeth very large and strong, yet less so than in the cats, and molar teeth the surfaces of which instead of being raised into cutting edges are depressed, tuberculated, and require a certain degree of lateral motion in the jaw to bring them into action.

In the seals a very different structure of the teeth is observed. The canines are not particularly large and prominent, as there are no hard substances to be cut or broken, and the molar

teeth-adapted neither for shearing on the one hand, nor on the other for grinding their food, either of which actions

Dentition of Seal.

would be unavailable in their particular case - are numerous, and furnished with several angular points extremely well-fitted for holding the slippery scaly surface of fish, and equally so for crushing them before they are swallowed.

The peculiar food of the rodents, which generally consists of hard vegetable substances, naturally required a very different dentition. Here the canine teeth, which would have been worse than useless, are suppressed; while the incisors, which play a very important part, are converted into powerful chisels. Their conformation is beautifully adapted to the purpose they have to fulfil: they are required to have a sharp edge, in order to make their way through tough vegetable substances, and they must at the same time be very strong and firm; this is effected by the principal substance of the tooth being composed of very tough ivory, with a plate of hard enamel in front only, which latter, wearing away more slowly, is always left as a sharp projecting edge. The molar teeth, which are separated from the incisors by a wide interval, are composed of alternate plates of enamel and ivory, which, wearing unequally, stand up in ridges, and give them a rasplike surface. The ridges are always transverse, or in a direction from side to side of the head; and as the jaw has considerable facility of moving backwards and forwards, it greatly increases the power of trituration. In the frugivorous species of the order, however, the surface of the molar teeth is raised into rounded tubercles-as in the squirrel, for instance; whilst in those animals which have any carnivorous tendency (as in the rat), they are raised into sharp points, thus bearing some resemblance to those quadrupeds which are wholly

carnivorous.

CRANIUM OF THE SHEEP.

343

The ruminants, who principally live on green leaves or tender shoots, naturally required a very different dentition from that

Skull of the Sheep, viewed laterally.

of the rodents, who have been specially appointed to devour the hardest substances, generally living upon the wood and bark of trees, as well as upon nuts and other shelled fruits. Here the lower jaw only is provided with six shovel-formed incisors, projecting almost horizontally, and pressing against the upper jaw,

which in nearly all cases is destitute of incisor teeth; their place being supplied by a kind of callous pad, a formation exceedingly

well adapted for plucking or nibbling the herbage. The canines are inconstant, and the molars, usually six on each side of both jaws, have flattened crowns surmounted by two double and irregularly crescentic folds of enamel, formed for affording the greatest possible extent of triturating surface. The lower jaw of the ruminants is not so broad as the upper one (as we see in the accompanying illustration), so that the surfaces of the upper molars project on both sides beyond those of the corresponding inferior rows; but both have been made to cover each other by the alternating lateral movements of the lower jaw to the right or to the left, a motion as admirably calculated for assisting the grinding or triturating process, as the hingelike and vertical motion of the lower jaw of the carnivora for shearing or cutting.

Several mammals are distinguished by the enormous develop ment of their canine teeth. Thus in the walrus they constitute formidable weapons of defence, and no less useful levers with which the unwieldy animal raises his huge body upon the iceblocks and precipitous shores where he loves to bask in the sun.

The long curved and sharp tusks of the wild-boar are capable of inflicting the most severe and painful wounds, and serve likewise for the digging-up of roots. In the narwhal only one of the upper canines projects in the shape of a formidable horn,

while the other remains in a rudimentary condition. This horn, which is harder and whiter than ivory, is from six to ten feet long, spirally striated throughout its whole length, and tapering to a point. We know but little of the habits of the narwhal, but have no reason to doubt that this powerful horn, which is restricted to the males, is an admirable weapon of defence. The upper incisors of the elephant, developed to an extraordinary size, form the tusks of this wonderful animal, and not only surpass other teeth in size, as belonging to a quadruped so enormous, Tusks of Walrus. but are the largest of all teeth in proportion to the size of the body. Their possessor uses them for ploughing up the earth in quest of nutritious roots, or as a protection

STOMACH OF THE SHEEP.

345

for his trunk, which when menaced he retracts between them, employing them at the same time to repel an aggressor. Thus in every deviation from the ordinary forms of dentition, we find that the attainment of some useful purpose was in view.

An examination of the digestive organs of the several orders of the mammalia, shows that in every case they are no less beautifully adapted to the peculiar food of the animal than the construction of the dental apparatus. The carnivora feeding on aliment which requires but little elaboration to convert it into nourishment, the whole process of digestion appears to be as rapid as possible, the stomach is simple and almost straight, the intestines short, and without any structure to retard the passage of the food. The ruminants, on the contrary, living on crude vegetable matters, containing but a small quantity of nutritious particles in proportion to their bulk, required a far more complicated alimentary canal for the elaboration of their food.

Here, therefore, the stomach consists not of one but of four distinct cavities. The first compartment, or paunch (b), is the largest of the four stomachs, and in the typical species, such as the sheep or ox, its internal surface is densely beset with prominent and pedunculated villosities. The second stomachal viscus, or the reticulum (c), is of much smaller dimensions than the paunch, and forms a kind of cul-de-sac between it and the third cavity. It is distinguished internally by the presence of a multitude of polygonal cells, and from this circumstance has been vulgarly denominated the honeycomb bug.