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 XXYI 
Reithrodon cuniculoides, Waterh., Proceedings of the Zoological Society of London for February 1837, p. 30. 
R. suprd griseus, jiavo-lavatus, pilis nigris intermixtis ; abdomine gulaque pcdlidk 
Jlavis; natibus albis ; pedibus albis ; auribus mediocribus, intus pilis jlavis, extiis 
pilis pallidb Jlavis, obsitis ; pone aures, nota magnd albescenti-jlavd ; caudd corpore 
breviore, suprd pallidb fused, subtus alba. 
Description. — Head rather large and arched ; ears moderate ; tail nearly as long 
