insect-jaws is very great, as many people have the misfortune to know. From before the time of Pharaoh, the biting insects have been a scourge to the farmer. Their voracity is awful; and, when urged by hunger, few substances can withstand their jaws. In countless hosts they ravage a country, and blast it as with the curse of Jehovah. It is stated that some caterpillars eat three to four times their weight of food every day. That beasts like the elephant and tiger are not voracious in proportion to their size is matter for congratulation. Fortunately, however, such ravenous insects eat only in the larval state; for most moths and butterflies exist on love, and none take any more substantial nourishment than the honey of flowers.

The common fly has minute teeth. Many grubs live within trunks of trees, gnawing immense galleries, and subsisting on the wood. The termites, or "white ants," devour whole houses, leaving only a shell fair to all external appearance, but crumbling at a touch. "At Tonnay-Charente the termites, having gnawed away the props of a diningroom without its being perceived, the flooring collapsed during a party, and the entertainer and his guests sank through." The larva of the giant sirex gnaws burrows in lead.

Teeth of mollusks and articulates are usually horny-that is, hardened skin, like the crust of the lobster and beetle. Sometimes they

are calcareous and sometimes siliceous. Those of the vertebrates are complex in structure and substance, and are regarded as the only true teeth. They consist of dentine or ivory, of which there are several kinds. In mammals and the highest reptiles, the dentine is surrounded by a sort of bone called cement; and the surface exposed to wear is capped or otherwise protected by enamel, the hardest of animal tissues. Teeth do not belong to the bony skeleton, but are developed by the lining membrane of the mouth, which, like the lining of the whole food-canal, is only a continuation of the skin. Hence teeth are classed with other skin appendages, as the nails and hair.

The teeth of fishes are extremely various in number, form, struc

ture, and method of attachment. They are almost entirely used for prehension, as nearly all fishes, like reptiles and birds, do not masticate but gulp down their food as quickly as possible. The exceptions, however, are interesting. Some rays and the cestracion of Australia have the jaws filled with teeth, flattened and joined together like blocks in a pavement, or like mosaic. These are used to crush sea-weed and mollusks. In a former geological age, this was the prevailing form of teeth in the whole order of sharks. The common carp has its teeth on the bones of the pharynx, and hence masticates its food in its throat. For this purpose the food is sent back after being swallowed. Some fishes are toothless, but most fishes have hundreds of teeth, frequently covering all parts of the mouth. The teeth of fishes and of reptiles are shed and replaced indefinitely.

FIG. 7.-SKULLS AND TEETH OF MUSKRAT (Fiber zibethicus). Size reduced. 1, cranium with upper incisors overgrown, due to loss of lower incisors; 2 a, skull and jaw with normal teeth; 26, lower incisor removed, to show its great length.

Reptilian teeth present no great variety. Toads, tortoises, turtles, and some lizards are entirely destitute of teeth. Frogs have teeth in the upper jaw. Those of serpents assist in swallowing the huge prey. The poison-fangs of venomous species present a peculiar and complex modification. They are fastened to movable bones which are worked

by the muscles moving the jaw. When the mouth is closed, the fangs lie on the gums; but in opening the mouth they are brought into a striking position. The muscles also press at the same time upon the glands which secrete the venom, and force the latter through a deep channel or canal in the fang.

Some birds of ancient times had true teeth placed in sockets in the jaws; but all modern birds depend wholly on the gizzard for mastication. This is literally a mill. It is formed of powerful muscles, has a horny lining, and pulverizes hard grain and indeed almost any substance by rubbing between the tough walls. To assist the grinding, the grain-eaters swallow gravel, bits of broken glass and crockery, metals, etc., which are pulverized in turn. One would suppose such

diet would injure the organ; but the tough lining yields without being cut. In birds the gizzard follows the stomach; that is to say, the food is not ground until after it is subjected to the gastric juice. This seems a reversal of the proper order, but the hard grain is more easily pulverized by rubbing after being softened by the solvent fluid. In other animals the gizzard, if a distinct cavity, usually precedes the true stomach. The power of this mill is proportional to the resistance of the food. Thus in flesh-eating birds the gizzard is weak.

Mastication is best exhibited in mammals and is almost entirely by means of teeth. Mammalian teeth are of three kinds: incisors or front cutting teeth; canines, which characterize flesh-eaters; and molars or masticators. They are placed in sockets in the bones, but always in a single row on the outer edge of the jaws, and are never renewed more than once.

The extreme numerical variation of mammalian teeth is found rather strangely in the same group, the order of whales. A river-dolphin of South America has the greatest number, two hundred and twentytwo; while the whalebone whale has no developed teeth, they being replaced with baleen-plates. The narwhal, or sea-unicorn, has, of two

in embryo, but a single tooth developed. This, however, is remarkable as being the longest in the animal kingdom. It is the left upper canine, and except in rare cases possessed by the male alone. Instances are recorded where both teeth were developed. This ivory tusk points directly forward in line with the body, perfectly straight, and sometimes attains the length of ten feet. The rough surface is spirally grooved as if the tusk were twisted. Various unsatisfactory conjectures have been offered regarding the object of this strange development; but, beyond its evident use as a weapon, its purpose is still a mystery.

The whole group of ant-eaters, and the sloths and armadillos, are quite destitute of teeth, on which account they are called edentates. Many of them have no teeth whatever; and when teeth are present

they are limited to the back part of the jaws, without enamel and rootless-all of which features help to rank the edentates very low among mammals. In place of teeth, the duck-bill-the lowest of mammalshas four horny plates, two in each jaw.

The average number of mammalian teeth is thirty-two-possessed by man, apes, and ruminants. But, the hog is the happy possessor of the typical number, forty-four; which honor is also shared by the opossum and mole. Man is the only living animal with an unbroken succession of teeth, and having the canines of the same height as the others.

The incisors of rodents are very interesting. They are the only prehensile organs of the gnawing mammals, and are exposed to severe wear. But, as the enamel is thicker and the dentine harder in front, the abrasion constantly produces a chisel-edge admirably fitted for gnawing hard substances. They must, however, be kept of a certain length; and to supply the loss by abrasion they are continually growing out from the base, being supplied by a permanent pulp. Their length in the jaw is great, to insure solidity without actual union with

the bone. The loss of an incisor results in the abnormal lengthening of the opposing one, which may finally interfere with mastication and cause starvation. Ruminants have no incisors in the upper jaw; the elephant has none in the lower jaw, but the tusks are upper incisors. An elephantine mammal of former geological time, the dinotherium, had incisor tusks in the lower jaw, pointing down and backward; while the extinct mastodon had tusks in both jaws.

The canines are intended for seizing and tearing prey and especially characterize carnivorous mammals. They are lacking in rodents and most herbivores, and are never more than four in number. In the apes they are very prominent; those of the gorilla nearly equaling a lion's in size. The tusks of the walrus are upper canines, as also are the terrible tusks of the wild hog. In the Malayan hog the upper

FIG. 10.-BABY ROUSSA, OR MALAYAN WILD HOG.

canines, instead of pointing downward, as would seem proper, grow upward through the integuments of the skull and curve backward, sometimes reaching the skull again. Their purpose is obscure. Although possessed in such degree by the male alone, the form precludes their efficient use as weapons. The lower canines also grow to enormous length and are directed outward, forming weapons which make the beast a formidable antagonist.

Herbivorous mammals have the molars flat on the grinding surface, and the enamel and cement disposed in plates and folds perpendicular to this face. Thus by the unequal wear of the tissues the acting surfaces are ridged and admirably adapted for grinding. They form an actual grist-mill, the stones of which never need any "picking." In fruit-eaters the crowns of the teeth are rounded. Insectivorous mammals have the teeth conical and fitting in opposing depressions. Those living on a mixed diet, as man, have the tubercles or cusps somewhat blunt, and suited either for crushing or cutting. In purely carnivorous mammals the molars have sharp edges fitted for cutting meat. They act like chopping-knives, or more accurately like scissors. Quite the only motion of the jaw is up and down, as flesh can not well be ground. Here we have a genuine "hash-mill." The backward and forward