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.