Rat du Brézil, Geoff.
M. suprà fuscus fulvo lavatus; lateribus capitis corporisque æquè ac abdomine auratis; gulâ pectoreque albis; pedibus pilis sordidè flavis tectis; auribus parvulis; caudâ caput corpusque ferè æquante; vellere longo, molli.
Description.—Head somewhat arched, and rather short; ears small; tail about equal in length to the head and body, measured in a straight line; tarsi large. Fur long, and rather soft; general colour deep golden yellow: on the upper surface of the head and the back, long glossy black hairs are thickly interspersed, and produce, with the admixture of the deep golden colour of the ordinary fur, a dark brown tint; chin, throat, chest, and rump, white; the hairs covering the upper surface of the feet are of a dirty yellowish white colour, and on the toes nearly white: ears densely clothed with longish hairs, those on the inner side chiefly of a deep golden colour, and those on the outer side brownish; the ears are partially hidden by the long fur of the head; tail sparingly clothed with hairs, above brown, and beneath brownish-white: the fur of the back is of a deep gray colour at the base, annulated with deep golden yellow near the apex, and blackish at the apex; the longer hairs are black; the hairs of the belly are pale gray at the base, and broadly tipped with golden yellow colour; the white hairs on the throat, chest, and rump are of an uniform colour—not tinted with gray at the root;—the hairs of the moustaches are black: the incisors of the upper jaw are of a deep orange colour, and those of the lower jaw are yellow: the thumb nail is truncated.
| In. | Lines. | ||
|---|---|---|---|
| Length | from nose to root of tail | 8 | 6 |
| of tail | 7 | 9 | |
| from nose to ear | 1 | 8 | |
| of tarsus | 2 | 0 | |
| of ear | 0 | 6½ |
Habitat, Bahia Blanca, (September.)
This species is nearly equal in size to the common rat (Mus decumanus). Of its skull[[23]] I possess but the anterior portion (see Pl. 33. fig. 3, a. and 3, b.): it appears to have been about the same size as that of M. decumanus, its proportions, however, are different: the nasal portion is broader and shorter, the ant-orbital outlet is rather smaller; the plate, forming the anterior root of the zygomatic arch, and which protects this outlet, has its anterior edge distinctly emarginated, and not nearly straight as in M. decumanus,—the zygomatic arch is stouter, the space between the orbits is narrower, the palate is more contracted, the incisors are much broader, less deep from front to back, and have the anterior surface more convex; the molar teeth are larger; the lower jaw (see Plate 34. fig. 12, a.) when compared with that of Mus decumanus also offers many points of dissimilarity; the principal differences consist in its greater strength, the comparatively large size and breadth of the articular surface of the condyles, the upright position of the coronoid process—a perpendicular line dropt from the apex of which would touch the posterior part of the last molar—and the great extent of the symphysis menti. In the form of the incisors, the more contracted palate, the great extent of the symphysis menti, and in fact in most of the points of dissimilarity, between the skull of the present animal and that of Mus decumanus, here pointed out, it will be perceived, there is an approach made to the Arvicolidæ.
The dimensions of the skull (so far as an imperfect specimen will allow of their being taken) are as follows:—
| In. | Lines. | ||
|---|---|---|---|
| Distance between front of incisors, (upper jaw) and the first molar tooth | 0 | 8 | |
| Longitudinal extent of the three molars on either side, taken together | 0 | 4¼ | |
| Length | of nasal bones | 0 | 7¼ |
| of incisive foramina | 0 | 4¼ | |
| Width | between orbits | 0 | 2½ |
| Length | of ramus of lower jaw | 1 | 1¼ |
Fig. 3, c, Plate 33, represents the molar teeth of the upper jaw. Fig. 3, d, those of the upper jaw.
“This rat was caught at Bahia Blanca where the plains of Patagonia begin to blend into the more fertile region of the Pampas. It lived in holes amongst the tussocks of rushes, on the borders of a small, still, brook; in its manner of diving and aquatic habits it closely resembled the English water-rat, (Arvicola amphibia.)”—D.