xv THE GERBILLES A75 
tion, and the lower incisors are worn to needle-hke points. The 
two molars are excessively minute, and thus the always large gap 
in the jaws is greatly exaggerated. It is suggested that this Rat 
is an insect-eater, but nothing positive is known. 
Sub-Fam. 4. Gerbillinae.—The Gerbilles form another sub- 
family, Gerbillinae, of the Muridae, or a family, according to some. 
The best-known genus is Gerbillus, including the Gerbilles proper. 
These animals are Old World in range, belonging to the three 
regions of that part of the world. There are a large number of 
species in the genus, over thirty. They have a Jerboa-like form, 
with rather long hind-lmbs and a long and hairy tail. But the 
hind- as well as the fore-feet are five-toed. The molar teeth have 
no trace of tubercles, but only transverse lamellae of enamel. 
Fic. 236.—Gerbille. Gerbillus aegyptius. ~x 4. 
The incisors are orange; they are white in Dipus. Gerbillus 
pyramidum is 90 mm. long, with a tail of 125 mm. The ears 
are long, 13 mm. The tail has longer hairs at the tip. 
Psammomys is in some respects different. The tail is shorter 
than in Gerbillus ; its length in an individual of 165 mm. was 
130 mm. As in Gerbillus there are four pairs of teats, two 
pectoral and two inguinal. This genus is exclusively Palaearctic 
in range. Jeriones has a range co-extensive with that of 
Gerbillus. 
Pachyuromys is an Ethiopian genus with a short tail. As 
the generic name denotes, the tail is not only short but thick 
and fleshy. 
Sub-Fam. 5. Otomyinae.—The allied genera, Otomys and 
Oreinomys, are Ethiopian. Otomys unisulcatus has a tail shorter 
than the body, the measurements of a female of this species being 
137 mm. with a tail of 87 mm. The ear is long, whence the 
name; it measured in this specimen 20 mm. 
