OBSERVATIONS ON THE RODENTIA. 187 



sive foramina rather large, long and narrow, situated partly 

 in the intermaxillaries and partly in the maxillary bones. — 

 Orbits large, extending far back, and leaving but a narrow 

 passage for the temporal muscle. Ant-orbital foramen very 

 large. The arch which incloses the ant-orbital foramen, and 

 separates this from the orbit, is formed by two bones, the su- 

 perior maxillary bone and the malar, the latter running pa- 

 rallel with the former, and articulating with the lachrymal 

 bone. The maxillary bone may be described as throwing 

 out two processes, one superior and one inferior, which unite 

 to form an arch. The superior process is thrown out from 

 the plane of the upper surface of the skull, and the inferior is 

 directed outwards from the plane of the palate, and is bifur- 

 cate, one portion being carried upwards to join the superior 

 process and form the arch, and the other portion, directed 

 backwards beneath the malar bone, assists in the formation of 

 the zygoma. Zygomatic arches slender and curved down- 

 wards, so that their lower boundary is below the level of the 

 palate, the hinder portion of the zygoma is horizontally com- 

 pressed. The glenoid cavity somewhat contracted and ob- 

 lique in its position, being directed forwards and inwards from 

 the root of the zygomatic process of the temporal bone, and 

 extending upon the sphenoid. Palatine process of palate 

 bone continuing the plane of the palate beyond the line of 

 the last molars. The inter-parietal bone is large and nearly 

 of a semicircular form. 



Lower jaw with the coronoid process rather small, the con- 

 dyloid curved inwards : the descending ramus (or posterior 

 coronoid process, according to Carus) is somewhat quadrate 

 and perforated, angular in Dipus, and has the lower and pos- 

 terior portion somewhat rounded in Alactaga. The symphy- 

 sis menti is of but small extent. 



My materials for drawing up the characters of the present 

 group are very limited : skulls of Dipus Mgyptius, and two 

 or three figures, are all I have at my command. M. F. Cu- 

 vier (in his ' Memoire sur les Gerboises,' &c. * ) has figured 

 the skull of a species of Dipus and that of an Alactaga, and 

 in both the descending ramus of the lower jaw is perforated. 



The skulls of the species of Dipus are remarkable for the 

 somewhat oblique direction of the glenoid cavity of the tem- 

 poral bone, and for the great developement of the auditory 

 bullce, which encroach upon and contract the occipital por- 

 tion of the cranium. A narrow band of the squamous por- 

 tion of the temporal bone is extended backwards over the 



1 See ' Transactions of the Zoological Society,' Vol. ii. pi. 24. 



