The thirteenth order of Mammalia is that of the Rodentia, or Rodent Animals, often spoken of as Glires, comprising the Mice, Rats, Squirrels, Rabbits, Hares, Beavers, etc.

The Rodentia are characterised by the possession of two long curved incisor teeth in each jaw, separated by a wide interval from the molars. The lower jaw never has more than two of these incisors, and the upper jaw very rarely; but sometimes there are four upper incisors. There are no canine teeth, and the molars and praemolars are few in number (rarely more than four on each side of the jaw). The feet are usually furnished with five toes each, all of which are armed with claws; and the hallux, when present, does not differ in form from the other digits. The testes pass periodically from the abdomen into a temporary scrotum, and the placenta is discoidal and deciduate.

The most characteristic point about the Rodents is to be found in the structure of the incisors, which are adapted for continuous gnawing - hence the name of Rodentia. The incisor teeth are commonly two in each jaw, and they grow from persistent pulps, so that they continue to grow throughout the life of the animal. They are large, long, and curved, forming segments of a circle (fig. 438, A), and are covered anteriorly by a plate of hard enamel. The back part of each incisor is composed only of the comparatively soft dentine, so that when the tooth is exposed to attrition, the soft dentine behind wears away more rapidly than the hard enamel in front. The result of this is that the crown of the tooth acquires by use a chisellike shape, bevelled away behind, and the enamel forms a persistent cutting-edge.

The gnawing action of the incisors is assisted by the articulation of the lower jaw, the condyle of which is placed longitudinally and not transversely, so that the jaw slides backwards and forwards. The molars, consequently, have flat crowns (fig. 438, B), the enamelled surfaces of which are always arranged in transverse ridges, in opposition to the anteroposterior movements of the jaw. The intestine is very long, and the caecum voluminous (rarely wanting). The brain is nearly smooth, and without convolutions. The orbits are not separated from the temporal fossae, and the eyes are directed laterally. The Rodents are almost all very small animals, and they are mostly very prolific. They subsist principally, if not entirely, upon vegetable matters, especially the harder parts of plants, such as the bark and roots. Many of them possess the power of building elaborate nests, and most of them hibernate. They are very generally distributed over the whole world, but no member of the order has hitherto been detected in rocks older than the Eocene Tertiary.

Fig. 438.   A, Side view of the skull of Sciurus (Cynomys) Ludovicianns; B, Molar teeth of the upper jaw of the Beaver (Castor fiber). (After Giebel.)

Fig. 438. - A, Side-view of the skull of Sciurus (Cynomys) Ludovicianns; B, Molar teeth of the upper jaw of the Beaver (Castor fiber). (After Giebel.)

In accordance with the number of the incisor teeth, the Rodents may be divided into the following two primary sections:

(1.) Simplicidentata. - Only two upper incisors, and these enamelled in front only. This division comprises all the Rodents except the Leporidae and Lagomydae.

(2.) Duplicidentata. - (Hares, Rabbits, and Calling Hares). Two rudimentary incisors behind the large central ones.

The order Rodentia comprises a very large number of families, which can be merely noticed here.

Fam. 1. Leporidae. - In this family are the Hares (Lepus timidus) and Rabbits (Lepus cuniculus), distinguished amongst the Rodents by the possession of two small incisors in the upper jaw, placed behind the central chisel-shaped incisors, so that there are four upper incisors in all. The molars and premolars are rootless, and the dental formula is:

i

2 - 2

c

0 - 0

; pm

3 - 3

; m

3 - 3

=

28.

1 - 1

0 - 0

2 - 2

3 - 3

The clavicles are imperfect. The fore-legs are furnished with five toes, and are considerably shorter than the hind-legs, which have only four toes. The two orbits communicate by an aperture in the septum. There is a short erect tail.

The common Hare (Lepus timidus) is dispersed over the whole of Europe, but is not met with in Northern Scandinavia, its place there being taken by the Mountain-hare (white in winter), which occurs commonly in Scotland. As a rule, the Hares occur in temperate regions, but some are found in Africa, and one species (Lepus glacialis) is a native of the Arctic regions, whilst the common American Hare (L. Ame?'icanus) extends from Canada to Mexico. The Rabbit (Lepus cuniculus) is also a native of temperate regions, but appears to thrive, to a more than average extent, in Australia.

Fam. 2. Lagomydae. - In the Calling Hares or Pikas (Lago-mys), which form this family, the legs do not differ much in size, there is no visible tail, and the clavicles are nearly complete. There are only five back teeth (instead of six) on each side of the upper jaw, but there are two rudimentary incisors besides the central ones. They resemble the Guinea-pigs in form, and are found in Russia, Siberia, and North America.

Fam. 3. Caviidae. - In this family the tail is rudimentary; the incisors are short; the back teeth are rootless; the clavicles are rudimentary or wanting; the feet are, typically, three-toed, and the claws are in the form of hoof-like nails. They are all South American. The Capybara (Hydrochoerus capybara) is the largest of living Rodents, attaining a length of three or four feet. It leads a semi-aquatic life, and has the feet incompletely webbed. The Cavia aperea has short legs and ears, and is believed to be the parent stock of the domesticated Guinea-pigs; while other species of Cavy are also found in South America.

The Agoutis and Pacas are sometimes separated as a distinct family (Dasyproctidae), having long incisors, molars at first rootless, but afterwards rooted, rudimentary clavicles, and five-toed fore-feet. The Agouti (Dasyprocta Aguti) is found in Guiana, Brazil, and Peru. Its fore-feet are five-toed, but the hind-feet have only three toes. The Paca (Coelogenys paca) has five toes on both the hind and fore feet. It has the zygomatic arches enormously inflated, the maxillary portions being hollowed out into chambers which are lined by mucous membrane, and open into the mouth, and the use of which is quite unknown. It inhabits Central and South America.