Skeleton of Coiiinwii Rat. 7 



backwards oii to the posterior edge of the lower jaw, aided by that 

 of the pterygoids, that the anteroposterior movement of the lower 

 jaw \vith its molar series upon that of the upper jaw is effected. 

 The glenoid cavity has, to allow of this movement, an anteropos- 

 terior directioji throughout the order, with the exception of the 

 Leporidae, and the unbroken molar series and the absence of canines 

 is characteristic of the order without even that exception. Though 

 the malar arch has a downward, rather than, as in Carnivora, an 

 outward curve, still the interzygomatic diameter is in all Rodents 

 the widest transverse cranial diameter. The temporal is never 

 separated from the orbital fossa; the cranial cavity is always much 

 compressed from side to side on a level with the optic foramina, 

 so as frequently to leave an interorbital fenestra by the fusion of 

 the two foramina into one, at a point a little behind that at which 

 the olfactory chamber succeeds the cerebral internally. 



The length of the tail and the number of the caudal vertebrae 

 vary much within the limits of this order, just as the external 

 concha of the ear and the characters of the integumentary system 

 do. But, in spite of the very various special habits of the animals 

 belonging to this order, the two pairs of limbs almost invariably 

 present the same ratio of development hder se, the hind limbs being 

 the stronger and longer pair. The tibia and fibula are anchylosed, 

 as they are here, more frequently than the ulna and radius. There 

 is, however, little tendency to anchylosis in the skeleton of the 

 Rodents ; in this specimen the posterior pair of sacral vertebrae are 

 not anchylosed with the anterior, with which the ilia articulate, 

 and the mandibular bones never throughout the order become an- 

 chylosed with each other at the symphysis of the lower jaw, in spite 

 of the great afflux of blood which their permanently growing inci- 

 sors bring into them. In the trunk we observe that the spines of 

 the dorsal vertebrae, from the largely developed spine of the second 

 dorsal to that of the tenth inclusively, point backwards, whilst 

 those of the six lumbar vertebrae and of the two last, the thirteenth 

 and the twelfth dorsal, point forward towards the vertical spine of 

 the eleventh dorsal, which has been called in consequence the 

 ' anticlinal ' vertebra. The anterior dorsal vertebrae diminish pro- 

 gressively in size as they are placed nearer to this vertebra, whilst 

 the vertebrae placed posteriorly to it, and markedly the transverse 

 processes of the lumbar vertebrae, increase in size as we pasb 



