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. 440), and the Voles (Arvicola); the latter represented by many species in both Europe and America. The molars of the Voles (fig. 439) are composed of alternating triangular prisms. Three species of Arvicola (the Water-rat, the Field-vole, and the Black Vole) are found in Britain.

Fig. 439.   Molar teeth of the Water rat (Arvicola amphibius).

Fig. 439. - Molar teeth of the Water-rat (Arvicola amphibius).

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 migration always terminating in the sea, and ending in the drowning of all that have survived the journey.

Fig. 440.   Common Hamster (Cricetus vulgaris).

Fig. 440. - Common Hamster (Cricetus vulgaris).

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. 12. Dipodidae. - 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 AEgypticus) of Africa and south-western Asia, which lives in societies and constructs burrows; the Jumping Hare (Pedetes Capensis) of South Africa, and the Jumping Mouse (Zapus or Meriones Hudsonicus) of North America.

Fam. 13. Myoxidae. - 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 (Muridae) on the one hand, or the Shrew-mice (Soricidae) 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. In form, the Dormice are Squirrel-like, with a long and hairy tail. There are four rooted molars on each side 5 the pollex is rudimentary; and the intestine is destitute of a caecum.

Fam. 14. Sciuridae. - 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 molars are rooted, five in number in the upper jaw on each side (the first being often deciduous), and four on each side of the lower jaw ; their crowns, when unworn, being tuberculate.

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 (about one hundred in number) 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 (Ardomys), unlike the true Squirrels, are terrestrial in their habits, and live in burrows, having short tails, thick bodies, and short legs. 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.

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 Sciuridae, Muridae, Myoxidae, and Octodontidae (?), have representatives in the Eocene, and the families of the Dipodidae, Castoridae,Hys-iriridae, Cavidae (?), and Lago-mydae in the Miocene deposits. The Leporidae 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 Palaeocastor are Miocene; Chalicomys is Pliocene; the great Trogontherium (fig. 441) is Pliocene and Post-pliocene, and the equally gigantic Castoroides of North America is also found in the Post-pliocene deposits.

Fig. 441.   Jaw of Trogontherium Cuvieri. Post pliocene.

Fig. 441. - Jaw of Trogontherium Cuvieri. Post-pliocene.