hunted, it leads a solitary life. When living in social communities the beavers build dams across the rivers, as well as habitations for themselves, by gnawing across the branches of trees or shrubs, and weaving them together, the whole being afterwards plastered with mud. In this last operation the tail, which is flattened and scaly, is employed very much as a mason uses his trowel. There is no doubt but that the Beaver shows extraordinary ingenuity in these and similar operations; but there can be equally little doubt as to the greatly-exaggerated stories which have been set afloat in this connection. The Beaver is hunted chiefly for the sake of the skin, but also for the substance known as castoreum. This is a fatty substance, secreted by peculiar glands, and employed as a therapeutic agent.

Fam. 9. Saccomyda.-This family comprises the so-called Pouched Rats and Gophers of North America, all of which have large external cheek-pouches. Some of them (Geomys and Thomomys) have the fore-feet greatly developed, and adapted for burrowing; whilst the so-called "Kangaroo-Rats" (Dipodomys) have very long hind-legs, and the fore-limbs are not specially developed.

Fam. 10. Murida.-The next family of Rodents is that of the Murida, comprising the Rats, Mice, and Lemmings. In this family the tail is long, always thinly haired, sometimes naked and scaly. The lower incisors are narrow and pointed, and there are complete clavicles. The hind-feet are furnished with five toes, the fore-feet with four, together with a rudimentary pollex.

The Rats (Mus rattus and Mus decumanus), the common Mouse (Mus musculus), the Field-mouse (Mus sylvaticus), and the Harvest-mouse (Mus messorius) are all well-known examples of this family, and are too familiar to require any description. The three first are also common in North America, though not indigenous. Closely allied to the true Rats are the Hamsters (Cricetus, fig. 384), and the Voles (Arvicola); the latter represented by many species in both Europe and America.

A less familiar example of this family is the Lemming (Myodes lemmus). This curious little Rodent is found inhabiting the mountainous regions of Norway and Sweden. It is chiefly remarkable for migrating at certain periods, generally towards the approach of winter, in immense multitudes and in a straight line, apparently in obedience to some blind mechanical impulse. In these journeys the Lemmings march in parallel columns, and nothing will induce them to deviate from the straight line of march.

The Gerbilles (Gerbillus), though closely related to the

Jerboas, are generally placed in this family. Here also may be placed the Musquash or Ondatra (Fiber zibethicus) of North

America, which leads a semi-aquatic life, and has the tail compressed, and the hind-feet partly webbed.

Fam. 11. Dipodida.-The next family of the Rodents is that of the Dipodidae or Jerboas, mainly characterised by the disproportionate length of the hind-limbs as compared with the fore-limbs. The tail also is long and hairy, and there are complete clavicles. The Jerboas live in troops, and owing to the great length of the hind-legs, they can leap with great activity and to great distances. They are all of small size, and inhabit Russia, North Africa, and North America. The best-known members of this family are the common Jerboa (Dipus Egypticus), which lives in societies and constructs burrows; the Jumping Hare (Pedetes Capensis) of South Africa, and the Jumping Mouse (Meriones Hudsonicus) of North America.

Fam. 12. Myoxida.-The members of this family are commonly known as Dormice, and they are often included in the following family of the Squirrels and Marmots. They only require to be mentioned, as they must not be confounded with the true Mice (Murida) on the one hand, or the Shrew-mice (Soricida) on the other; the latter, indeed, belonging to another order (Insectivora). The common Dormouse (Myoxus avellanarius) is a British species, and must be familiarly known to almost everybody. No species of this family have yet been described from the New World.

Fam. 13. Sciurida.-This is one of the most characteristic and familiar of the divisions of the Rodents, and it comprises the true Squirrels, the Flying Squirrels, and the Marmots.

The true Squirrels (Sciurus) are familiarly known in the person of the common British species (Sciurus vulgaris), and the equally common Grey Squirrel (S. cinereus) of the United States. Numerous species more or less closely allied to these occur in other countries, and they are especially abundant in North America.

In the genera Pteromys and Sciuropterus, or Flying Squirrels, there is a peculiar modification by which the animal can take extended leaps from tree to tree. The skin, namely, extends in the form of a broad membrane between the hind and fore legs, and this acts as a kind of parachute, supporting the animal in the air. There is, however, no power whatever of true flight, and the structure is identically the same as what we have previously seen in the Flying Phalangers (Petaurus), which take the place of the Flying Squirrels on the Australian continent. The Flying Squirrels are found in southern Asia, Polynesia, the north-east of Europe, Siberia, and North America.

The Marmots (Arctomys), unlike the true Squirrels, are terrestrial in their habits, and live in burrows. Various intermediate forms, however, are known, by which a transition is effected between the typical Squirrels and the Marmots. Such, for example, are the Ground Squirrels (Tamias) of Europe, Asia, and North America. There are numerous species of this family inhabiting various parts of Europe and northern Asia, and generally distributed over the whole of North America. Good examples are the Alpine Marmot (A. Alpinus) of Europe, and the Prairie Dog (Cynomys Ludovicianus) of North America.

The Pouched Marmots (Spermophilus) have cheek-pouches, and are widely distributed over North America, northern Europe, and northern Asia.

Fam. 14. Spalacida.-The animals comprised in this family are generally known as "Mole-rats." They have a stout body, and short legs, the feet being five-toed, and adapted for burrowing. The external ears are small or wanting, and the eyes are extremely minute or rudimentary. The Mole-rats (Spalax, Georychus, &c.), are exclusively found in the Old World, and principally in Asia, Africa, and south-eastern Europe.

As regards the distribution of the Rodentia in time, very many fossil forms are known, the oldest appearing in the Eocene Tertiary, but the extinct forms offer few points of special interest. The families of the Sciurida, Murida, Myoxida, and Octodontida (?), have representatives in the Eocene, and the families of the Dipodidae, Castorida, Hystricida, Cavida (?), and Lagomyde in the Miocene deposits. The Lepor

ida do not seem to have made their appearance earlier than the Pliocene. Amongst the fossil Rodents perhaps the most

interesting are the extinct genera of Beavers. Of these the genera Steneofiber and Palaocastor are Miocene; Chalicomys is Pliocene; the great Trogontherium (fig. 385) is Pliocene and Post-Pliocene, and the equally gigantic Castoroides of North America is

Post-Pliocene.

cene deposits.

CHAPTER LXXVIII.

CHEIROPTERA.

ORDER XIV. CHEIROPTERA.* This order is undoubtedly "the most distinctly circumscribed and natural group" in the whole class of the Mammalia. In many respects, however, it would be advantageous to regard the Cheiroptera as a suborder of the next order (namely, the Insectivora) specially modified to lead an aerial life; just as the Pinnigrada are regarded as a mere section of the Carnivora specially modified to suit an aquatic life.

The Cheiroptera are essentially characterised by the fact that the anterior limbs are longer than the posterior, the digits of the fore-limb, with the exception of the pollex, being enormously elongated (fig. 386). These elongated fingers are united by an expanded membrane or "patagium," which is also extended between the fore and hind limbs and the sides of the body, and in many cases passes also between the hind-limbs and the tail. The patagium thus formed is naked, or nearly so, on both sides, and it serves for flight. Of the fingers of the hand, the pollex, and sometimes the next finger as well, is unguiculate, or furnished with a claw; but the other digits are destitute of nails. In the hind limbs all the toes are unguiculate, and the hallux is not in any respect different from the other digits. Well-developed clavicles are always present, and the radius has no power of rotation upon The Cheiroptera were placed by Linnæus in his order Primates, which contained also the Lemurs, the Apes, and Man.

the ulna.

The

The mammary glands are two in number, and are placed upon the chest. There are teeth of three kinds, and the canines are always well developed. The molars are tuberculate or grooved in the frugivorous forms, and cuspidate in the insectivorous species. The ulna is sometimes quite rudimentary. bones are not pneumatic. The testes are abdominal except during the breeding season. The stomach is complex and the intestine long in the fruit-eating Bats; but the reverse of this obtains amongst the insectivorous forms. The Cheiroptera are cosmopolitan in their distribution, and the oldest known species is from the Eocene Rocks.

The Bats are all crepuscular and nocturnal in their habits, and are sometimes carnivorous, sometimes frugivorous. The eyes are small, but the ears are very large, and their sense of touch is most acute. During the day they retire to caves or crevices amongst the rocks, where they suspend themselves by means of the short thumbs, which are provided with curved claws. In their flight, though they can fly in the genuine and proper sense of the term, and can turn with great ease, they are by no means as rapid and as active as are the true birds. The tail is sometimes short, sometimes moderately long, and is usually included in a continuation of the leathery patagium, which stretches between the hind-legs, and is termed the "inter-femoral membrane." The body is covered with hair, but the patagium is usually hairless, or nearly so. Most of the Bats hybernate.

The Cheiroptera are conveniently divided into the two sections of the Insectivora and Frugivora, according as the diet consists of insects or of fruits.

SECTION A. INSECTIVORA (Microcheiroptera).-In this section are the four families of the Vespertilionida, Rhinolophida, Noctilionida, and Phyllostomida.

Fam. 1. Vespertilionida. In this family are the ordinary Bats, distinguished by having a dentition very like that of the order of the Insectivorous Mammals, the molar teeth being furnished with small pointed eminences or cusps, adapted for crushing insects, and the incisors being of small size. The nose is not furnished with leaf-like appendages, and the tail is usually elongated, and enclosed in a large inter-femoral membrane. About fifteen species of this family have been described as British, but of these only two are at all common. Of these two, the Pipistrelle (Vespertilio pipistrella) is the commonest species, occurring over the whole of Britain. The long-eared Bat (Plecotus auritus) is also not uncommon, and is distinguished by its greatly elongated ears, which are con