Genus—REITHRODON.[[25]]
Dentes primores ²⁄₂; inferioribus acutis, gracilibus, et anticè lævibus; superioribus gracilibus, anticè longitudinalitèr sulcatis.
Molares utrinque ³⁄₃ radicati; primo maximo, ultimo minimo: primo superiore plicas vitreas duas externè et internè alternatìm exhibente; secundo, et tertio, plicas duas externè, internè unam: primo inferiore plicas vitreas tres externè, duas internè; secundo, plicas duas externè, unam internè; tertio unam externè et internè, exhibentibus.
Artus inæquales: antipedes 4–dactyli, cum pollice exiguo: pedes postici 5–dactyli, digitis externis et internis brevissimis.
Ungues parvuli et debiles. Tarsi subtùs pilosi.
Cauda mediocris, pilis brevibus adpressis instructa.
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.