MAMMALIA. 69 



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 dif- 

 ferences 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 cuniculoides. 



Plate XXVI 



Reithrodon cuniculoides, Waterh., Proceedings of the Zoological Society of London for February 1837, p. 30. 



R. suprd griseus, flavo-lavatus, pilis nigris intermix lis ; abdomine guldque pallide 

 flavis; natibus albis ; pedibus albis ; auribus mediocribus, intiis pilis flavis, extiis 

 pilis pallide flavis, obsitis ; pone aures, nota magna albescenti-flavd ; caudd corpore 

 breviore, supra pallide fused, sub tits alba. 



Description. — Head rather large and arched ; ears moderate ; tail nearly as long 



