the Beaver, or again, into pouches opening near the anus, as in the Hare, Agouti, and Jerboa.

The integument is generally thin, and the panniculus carnosus (the sheet of muscle underlying the skin) rarely much developed. The fur varies exceedingly in character. Thus it may be very fine and soft, as in the Chinchillas and Hares, in others more or less replaced by spines on the upper surface, as in the SpinyRats and Porcupines; in several genera, as in Xerus, Acanthomys, Platacanthomys, Echinothrix, Loncheres, and Echinomys, the spines are flattened. In the muscular structures the chief peculiarities are noticeable in the comparatively small size of the temporal muscles, and in the great double masseters (Fig. 194), which are the principal agents in gnawing; the digastrics also are remarkable for their well-defined central tendon, and in many species their anterior bellies are united between the mandibular rami; the cleidomastoid generally arises from the basioccipital, and the pectoralis major is connected with the latissimus dorsi; in the Porcupines and Hares the tendons of the flexor digitorum longus and flexor hallucis longus are connected in the foot, while in the Rats and Squirrels they are separate, and the flexor digitorum longus is generally inserted into the metatarsal of the hallux.1

Rodents are tolerably well represented in a fossil condition from the period of the Upper Eocene, while if Decticadapis, of the Lower Eocene of Rheims, is rightly referred to it the order dates from the oldest Tertiary. All the fossil forms at present known are, however, essentially true Rodents, and afford no clue as to the relations of the order with other mammals. The remote affinities of the Rodents to the Proboscidea, as well as their more marked resemblances to Typotherium, have been already mentioned. Whether there is a real genetic affinity (as Professor Cope suggests) with the Tillodontia cannot be decided with the evidence at present available.

Suborder SIMPLICIDENTATA.

Only one pair of upper incisors, having their enamel confined to their front surfaces. Incisive foramina moderate and distinct; fibula not articulating with the calcaneum. Testes abdominal, and descending periodically only into a temporary sessile scrotum.

Section SCIUROMORPHA.

Zygomatic arch slender, chiefly formed by the jugal, which is not supported by a long maxillary process extending backwards beneath it; postorbital processes of frontal present or absent; 1 See G. E. Dobson, Journ. Anat. Phys, vol. xvii.

infraorbital opening small (except in Anomalurus); mandible with the angular part arising from the inferior surface of the bony socket of the lower incisor; clavicles well developed; fibula distinct.

Family ANOMALURIDE.

Arboreal forms, having their limbs connected by a cutaneous expansion supported by a cartilaginous process arising from the olecranon; tail long and hairy, with large imbricated scales on its

FIG. 197.-Anomalurus fulgens. From Alston, Proc. Zool. Soc. 1875.

inferior surface near the root; sixteen pairs of ribs; no postorbital processes on the frontals; p; molars not tuberculate, with transverse enamel-folds. Confined to the Ethiopian region.

Anomalurus, with several species from West and Central Africa, alone represents the family. The peculiar caudal scales, which evidently assist the animal in climbing, and the position of the cartilaginous support of the parachute, are well shown in Fig. 197. All the species but two are from Western Africa; A. orientalis occurs near Zanzibar, and A. pusillus is from the equatorial regions of that

1 Waterhouse, Proc. Zool. Soc. 1842, p. 124.

continent. According to Mr. O. Thomas,1 the latter "little animal is most nearly allied to the West-African A. beecrofti, but differs from that species in its duller and less yellow upper side, in the entire absence of rufous on its neck and belly, and, as from all the other described species, in its diminutive size."

Arboreal or terrestrial forms, with cylindrical hairy tails, with

FIG. 198.-Lateral view of skull of American Marmot (Arctomys monax).

hairy. Cosmopolitan (excluding Australian region).

This subfamily includes the true Squirrels, of which seven existing genera are usually recognised.

Sciurus.2-Tail long and bushy; ears generally well developed,

True Squirrels are found in most of the temperate and tropical regions of the world, exclusive of Madagascar and the Australian region. They are, however, most abundant in the Malayan part of the Oriental region, and attain their largest size and most brilliant coloration in the tropics. Their size is very variable, so that whereas S. soricinus, of Borneo, is no larger than a Mouse, S. bicolor, of the Malayan region, is nearly as large as a Cat. The common

FIG. 200.-Burmese Squirrel (Sciurus pygerythrus). After Anderson.

English Squirrel (S. vulgaris) is found over the whole of the Palearctic region, reaching in one direction from Ireland to Japan, and in the other from the north of Italy to Lapland; its remains occur in the Norfolk "Forest-bed." In the Malayan region "nearly all the numerous species are brilliantly marked, and many are ornamented with variously coloured longitudinal stripes along their bodies. One of the commonest and best known of the striped species is the little Indian Palm-Squirrel (S. palmarum), which in large numbers runs about every Indian village. Another Oriental species (S. caniceps) presents almost the only known instance among mammals of the

temporary assumption during the breeding season of a distinctly ornamental coat, corresponding to the breeding-plumage of birds. For the greater part of the year the animal is of a uniform gray colour; but about December its back becomes a brilliant orangeyellow, which lasts until about March, when it is again replaced by gray. The Squirrel shown in Fig. 200 is a native of Burma and Tenasserim, and is closely allied to S. caniceps, but goes through no seasonal change of colour.

"The number of species in the genus is about 75, of which 3 belong to the Palearctic, 15 to the Ethiopian, about 40 to the Oriental, and 16 to the combined Nearctic and Neotropical regions (Thomas).

Fossil species referred to Sciurus are found in the European Tertiaries down to the Phosphorites of Central France, while others occur in the White River Miocene of the United States.

Rhithrosciurus.-A very striking Squirrel, confined to Borneo, and as yet only known from three or four examples, has been separated generically under this name. The general shape of its skull is very different from that of other Squirrels; but its most peculiar characteristic is the presence of from seven to ten minute parallel vertical grooves running down the front face of its incisors; no other Squirrel having really grooved incisors, and no other member of the whole order incisive grooves resembling these. Its premolars number, and its molars are simpler and less ridged than in the other genera. This Squirrel (R. macrotis) is far larger than the English, with an enormously long bushy tail, long tufted ears, and black and white bands down its sides.

Xerus.2-Fur coarse and spiny. Claws long and comparatively straight. Ear-conchs minute or absent. Skull with the postorbital processes short and directed backwards, the bony palate prolonged considerably behind the tooth-row, and the external ridge on the front face of the anterior zygomatic root more developed, and continued much farther upwards than in Sciurus. Premolars : molars as in Sciurus. Mammæ two. This genus contains four wellmarked species, known as Spiny Squirrels, all natives of Africa. They are terrestrial in their habits, living in burrows which they dig for themselves. X. getulus, a striped species of North Africa, has much the size and appearance of the Indian Palm-Squirrel ; all the others are a little larger than the English Squirrel.

Tamias. All the members of this genus are characterised by the possession of internal cheek-pouches, and by their style of coloration; being ornamented on the back with alternate light and dark bands. Their skulls are slenderer and lighter than those of the

1 Gray, Ann. Mag. Nat. Hist. ser. 3, vol. xx. p. 272 (1867).
2 Hemprich and Ehrenberg, Symbol. Phys. Mamm. vol. i. (1832).
3 Illiger, Prodromus Syst. Mamm. p. 83 (1811).