6 DESCRIPTIONS OF PREPARATIONS. 



process of the maxillary as round a pulley to an insertion just below the 

 socket of the mandibular premolar, very strongly with the temporal muscle 

 in moving the lower jaw in a vertical direction, and bringing its incisors 

 into play upon those of the upper jaw ; whence probably the inverse ratio 

 which has been observed to obtain between the temporal and the antorbital 

 fossae is to be accounted for. The masseter muscle arises from nearly the 

 whole length of the malar arch, which is made up ordinarily of the malar 

 process of the maxillary, of the malar bone, and of the malar process of the 

 squamosal, and sometimes of the lacrymal also. It is by the contraction of 

 those of its fibres which pass backwards on to the posterior edge of the 

 lower jaw, aided by that of the pterygoids, that the anteroposterior move- 

 ment of the lower jaw with its molar series upon that of the upper jaw is 

 effected. The glenoid cavity has, to allow of this movement, its long axis 

 running anteroposteriorly as in all Rodents except Leporidae, and as in the 

 Mesotheriidae ; whilst the unbroken molar series and the absence of canines 

 are characteristic of the entire order without any exception. Some involu- 

 tion of the angle of the lower jaw, which resembles that observable in the 

 Marsupialia, and the considerable size of this portion of the bone are points 

 worthy of note as being present in many Rodents. 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 inter 

 se, the hind limbs being the stronger and longer pair. The tibia and fibula 

 are anchylosed here and in Leporidae^ but not in Sciuromorphi nor Hystri- 

 comorphi. 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 anchylose, as they do in Pro- 

 boscidae, Suidae, and Perissodactyla, at the symphysis, in spite of the 

 great afflux of blood which their permanently growing incisors 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 



