220 Zoological Society. 
was exhibited: it differs chiefly in being of a smaller size, and in having 
the fur somewhat mottled with whitish and in parts rust colour. 
Mr. Waterhouse exhibited a new species of Rodent from the river 
Gambia, constituting a most interesting link between the genera 
Mus and Cricetus: like the first of these genera, it has a long scaly 
tail, but it resembles the Hamsters in possessing large cheek- 
pouches. In the number of its molar teeth and the form of the skull 
it presents all the most common characters of the Muride, as defined 
by Mr. Waterhouse in the ‘ Magazine of Natural History *.’ 
The skull comparéd with that of the Common Rat (Mus decuma- 
nus, Auct.) differs chiefly in having the nasal portion more elongated : 
the anterior root of the zygoma, as in that animal, is in the form of 
a thin plate, but this plate is less extended in its antero-posterior 
direction, ‘is directed obliquely outwards and upwards, and leaves a 
tolerably large and nearly round ant-orbital opening, thus differing 
from the Common Rat, in which the lower portion of this opening 
is in the form of a vertical slit: the zygomatic arch is less extended 
in the longitudinal direction, the incisive foramina are much smaller, 
and the auditory bullz are rather smaller in proportion. ‘The molar 
teeth are rooted; the foremost of these teeth in either jaw is the 
largest, and the posterior one the smallest : in the upper jaw, as in 
Mus, the molars present a central row of larger, and two lateral rows 
of smaller tubercles ; and the molars of the lower jaw have two prin- 
cipal rows of tubercles ; there are however some slight modifications 
in the structure of these teeth, which should be noticed. The front 
molar of the upper jaw has three central tubercles, three smaller ones 
on the outer side and two on the inner side, and besides these there 
is a small ninth tubercle on the posterior part of the tooth, which is 
not observed in the Black and Common Rats; the second molar has 
two small extra tubercles, one in front and cne behind; the crown 
of this tooth therefore presents eight instead of six tubercles, as in 
Mus proper, and the last molar possesses one extra small tubercle, 
which is placed on the anterior and outer part of the tooth. The 
molars of the lower jaw very closely resemble those of Mus decumanus. 
In the form of the lower jaw the present animal differs from that 
last mentioned, chiefly in the greater breadth of the descending ra- 
mus or angle, which is moreover somewhat raised, and so far ap- 
proaches the Hamsters. 
The name Cricetomys was proposed for this new subgenus, and 
that of Gambianus to distinguish the species, and to indicate the lo- 
cality in which it was first discovered. ‘The principal characters 
may be thus expressed :— 
Subgenus ad genera Cricetus et Mus dicta affine, et inter heec me- 
dium locum tenens. Criceto simile quoad saccos buccales, Muri 
simile quoad formam corporis et caudz ; hac perlonga et pilis brevibus 
vestita, inter quos squame in more annulorum posite videntur. Pe- 
des ut in Mure. 
Dentes fere ut in Mure. Jncisores compressi; molares radicati, 
3—3 
3-3" 
* Vol. iii. p. 275. 
