278 OBSERVATIONS ON THE RODENTIA. 



the ant-orbital nerve passes. Here the ant-orbital foramen 

 constitutes a tolerably large rounded opening, situated near 

 the upper surface of the skull, and also near the anterior an- 

 gle of the orbit. The broad spherical condyle also removes 

 this genus from the typical rats. In Mus Braziliehsis, how- 

 ever, we may perceive an approach to this spherical form of 

 the condyle. The great size and strength of the incisors in 

 Nyctocleples, require a corresponding development of the 

 temporal and maseter muscles; hence the great width of the 

 temporal fossae, and strength of the zygomatic arches, charac- 

 ters which exist in a minor degree in the common hamster. 



The skulls of upwards of forty species of the family Muri- 

 dce have been examined by me, and among these were crania 

 of the following genera. Mus, Gerbillus, Psammomys, Rei- 

 throdon, Hydromys, Cricetus, Sigmodon, Neotoma, Hapalotis 

 and Rhizomys. 



The skull of the Mus giganteus has been selected to exhi- 

 bit the most common form observable in the present group, 

 and the skulls of Psammomys obesus and Gerbillus brevicau- 

 datus have been drawn to show the approach made by these 

 species to those of the preceding family (the Gerboidce). It is 

 not only in the general form of the skull, with its narrow and 

 elongated nasal bones, that this affinity is evinced, but what 

 I consider more important, in the form of the descending ra- 

 mus of the lower jaw. In several of the specimens from 

 which M. F. Cuvier's figures of the Gerbilli (in the memoir 

 before referred to) were taken, it appears that this portion of 

 the lower jaw was imperfect, but where this was not the case, 

 they are all represented as having the upper posterior angle 

 of the descending ramus acute and elongated, as in the Ger- 

 boidce. 



The principal difference between the Gerboidce and the 

 Gerbilli consists in the size of the ant-orbital foramen, but 

 in either group this varies considerably, hence in all proba- 

 bility the discovery of other species will render it necessary 

 to merge the gerboas into the Muridce. I have thought it 

 desirable however, for the purpose of drawing attention to 

 the various modifications observable in the crania of these 

 animals, to separate these sections, and also to separate the 

 My oxides from the Muridce, although in so doing I may give 

 a name to groups which really are not distinct. 



The genus Psammomys of M. Ruppell is evidently an off- 

 shoot (if I may so term it) of the Gerbilli. A skull figured 

 by M. F. Cuvier, 1 as Gerbillus ? very closely resem- 

 bles that of Psammomys obesus. 



1 See Transaetions of the Zoological Society, vol. ii. pt. 2, pi. 26, fig. 1 & 2 



