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 Muridee, as defined 

 by Mr. Waterhouse in the • Magazine of Natural History*.' 



The skull compared 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 bullae 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 one 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 haec me- 

 dium locum tenens. Criceto simile quoad saccos buccales, Muri 

 simile quoad formam corporis et caudse ; hac perlonga et pilis brevibus 

 vestita, inter quos squamae in more annulorum positae videntur. Pe- 

 des ut in Mure. 



Dentes fere ut in Mure. Incisores compressi; molares radicati, 

 3-3 

 3 — 3* 



* Vol. iii. p. 275. 



