GERBILLUS MONTANUS. 
tudinal groove along its centre; those of the lower jaw long, slender, and 
plain. The molars are six in each jaw, three on each side, the front one of 
upper jaw is largest, and its crown consists of three transverse divisions, each 
division a sort of tube, the anterior and posterior sides of which are nearly in 
contact, and only connected with the adjoining division at a point about equi- 
distant between their inner and outer sides; the second and third teeth consist 
of two divisions constituted in the same manner as the first, the last division 
of the third tooth being very small and subtriangular. The molars of the lower 
jaw are similai'ly constructed. Length of the ossa nasi 7 lines ; distance between 
incisors of upper jaw and base of first molar 4j lines ; length of the row of 
molars 2§ lines. For representations of the Teeth, &c., see Plate XXXVII. 
This species inhabits the summit of hills in the country to the north of the Orange River 
towards its sources, and is generally found in situations devoid of shrubs, where the grass is 
short. Its principal operations are carried on during the night, and under the cover of dark- 
ness it seeks its food, forms its burrows, and changes its residence. 
The incisor teeth of the upper jaw are much larger in this species than in Gerbillus Afer, 
and the distance between the incisors and first molar is less ; hence the muzzle is considerably 
shorter, and consequently, the head appears more clumsy : there is also a difference as regards 
the fur; it is more closely set and better adapted to protect against the greater cold to which 
it is necessarily exposed. The nails of the fore feet also furnish diagnostic characters of the 
animal, as will be seen by comparing the descriptions of those organs in the two species ; the 
nails of G. montanus are vertically more compressed and like those of G. tenuis. 
