There are very rarely any vertebrae with unanchylosed ribs anteriorly to the first dorsal vertebrae. The jaws are ordinarily dentigerous, but teeth are never found elsewhere than upon the mandibular, maxillary, and intermaxillary bones; the grinding teeth very frequently have more than a single root or fang, a method of implantation never observed in any other class.
The most distinctive character of the Rodent order is the possession of the pairs of scalpriform incisors in the upper and lower jaws, from which their name is taken. There is a single pair of incisors in the upper jaw in all Rodents, except the Leporidae, which are hence called 'Duplicidentatil as having two pairs placed one behind the other, the hinder pair being the smaller. In the lower jaw there is a single pair only in all living Rodents, without exception. The upper incisors form a larger segment of a smaller circle, the lower a smaller segment of a larger circle. The peculiarities of their growth, which goes on uninterruptedly during the life of the creature from a persistent pulp, and of their functions, entail changes of great importance in the conformation both of the skull and of particular bones. The intermaxillaries, in relation with which the upper incisors are first developed, and which form a great part of their permanent sockets, are larger in relation to the rest of the skull and of the animal than in perhaps any other mammals; - they form the whole, or nearly the whole, of the sides and under surface of the bony snout, and in all living Rodents, as in the Elephants, they interpose between the nasals and the maxillaries, whilst failing themselves to reach the lacrymals.
The maxillary bone gives origin on the concave surface of its malar process to a large part of the masseter muscle, but a more deeply placed part of the muscle passes behind or inside of that process and takes origin from the sides of the snout. The presence of this deep head to the masseter is peculiar to but by no means constant in Rodents, varying with the infra- or ant-orbitally placed canal through which it passes. It co-operates, by passing round the back of the malar 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 movement 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 involution 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 the vertical spine of the eleventh dorsal, which has been called in consequence the 'anticlinal' vertebra.
The anterior dorsal vertebrae diminish progressively 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 pass backwards from it towards the sacrum. Well-marked and distinct anapophyses and metapophyses are developed on the anticlinal vertebra, and are to be seen on the succeeding vertebrae nearly or quite up to the sacrum. The direction of its spine relatively to those of the other vertebrae in front of and behind it, causes it to be the point of greatest mobility in the trunk. Points of less striking proportions, but more or less distinctive of, and universal in, the order are presented in the skull by the presence of an interparietal bone; by a vacuity in the skull walls for the blood to pass out from the lateral sinus, either as here by a conjugate foramen between the squamosal and the periotic, or by a foramen in the squamosal itself, the so-called 'canalis temporalis;' by the development of the post-auditory process of the squamosal into a lamina of bone, which may reach as far back as the occipital, but serves always to keep the tympano-periotic, with which it never anchyloses, in place; and, finally, by the smallness of the angle formed by a line drawn from the posterior edge of the supraoccipital on to the basicranial line.