_Caput magnum, fronte convexo: oculis magnis: auribus mediocribus._ The present genus according to my views belongs to the family _Muridæ_. The modifications of structure which have led me to separate it from the genus _Mus_ are as follows: _External characters._—The most conspicuous points of distinction between the external characters of _Reithrodon_ and _Mus_ (if we regard _M. rattus_, _M. decumanus_ or _M. musculus_ as typical examples of that genus,) consist in the arched form of the head, the large size of the eyes, the stout form of the body, and the upper incisors being grooved. The ears, tail and feet are more densely clothed with hairs, and the tarsus is covered with hair beneath,—at least the hinder portion. _Cranium._—The skulls of the species of the present genus differ from those of the species of _Mus_ in being proportionately shorter and broader, and more arched; the facial portion of the skull is larger, compared with the cranial, the space between the orbits is narrower, and the orbits are larger; the palate is narrower and the incisive foramina are more elongated and larger. The pterygoids approximate anteriorly, so that the posterior _nares_ are greatly contracted. As in the genus _Mus_ the anterior root of the zygomatic arch is directed upwards from the plane of the palate, and forwards in the form of a thin plate, protecting an opening behind, which leads into the nasal cavity, and also forming the outer boundary both of the ant-orbital foramen, and a second opening whose outlet is directed upwards. This thin plate, however, is narrower than is usually found in the genus _Mus_. The most striking differences observable in the lower jaw consist in the smaller size of the coronoid process, and its being curved outwards; the condyloid process is narrower, and the angle of the jaw, or descending ramus, approaches more nearly to a quadrate form—the posterior edge of the jaw is more deeply emarginated. _Dentition._—The incisors are narrow and compressed as in the genus _Mus_, but they are less deep from front to back; those of the upper jaw (Plate 33. fig 2. _b._) have each a distinct longitudinal groove, which is situated nearer to the outer than to the inner edge of the tooth. Close to the inner edge of each of these teeth an indistinct second longitudinal groove may be seen by means of a lens. The lower incisors are nearly equal in width to the upper. The crowns of the molar teeth in the young _Reithrodon_ are higher than in _Mus_, and they are rootless; in the adult animal, however, they possess distinct roots. The folds of enamel form sigmoid flexures, are closely approximated to each other, and those of the opposite sides of the tooth meet. 1. REITHRODON CUNICULOÏDES. PLATE XXVI.