The skeleton of the Rabbit differs from that of the Rat and many though not all other Glires Myomorphi, not merely in such points as its larger absolute size, the incompleteness 1 of its clavicles, the absence save in rudiment of a hallux, the unguiculate character of its pollex, the number and rootlessness of its molars, and the smaller number of its caudal vertebrae, but in many points of greater morphological importance than any of these. Some of these latter points show that the suborder Lagomorphi is more closely allied than the Myomorphi to certain lower forms of Vertebrata; others indicate more clearly than is seen in the Myomorphi that a certain affinity exists between the Rodentia s. Glires and the large Ungulate Mammalia; whilst others may have their connecting character expressed by saying that the distinctive peculiarities of the Rodent type are not so sharply pronounced in this as in the other suborder already mentioned, or indeed in either of the two other suborders of Glires, the Hystricomorphi and the Sciuromorphi.
1 The older zoologists (e.g. Fischer, Synopsis Mammalium, 1829, pp. 286, 366; Catalogue of the Royal College of Surgeons, Part iii. 1831, pp. 79, 87) divided the order into the two sections of Glires claviculis completis, saepe validissimis, and Glires claviculis nullis aut imperfectis. The inadequacy of this basis of classification may be judged of by the fact that the tail-less Hares (Lagomyes), which form a subfamily, as shown by Pallas, Glires, p. 28, closely allied to the true Hares, have complete clavicles. In the Rabbit no trace of the clavicle is visible at birth (see Parker, Shoulder-Girdle, PI. xxv. Figs. 1, 2, pp. 207-210; Flower, Osteology of Mammalia, p. 229), though it becomes developed before adult life. In the human subject, on the other hand, the clavicle ossifies before any other bone in the developing foetus.
Among the last of these three sets of peculiarities may be mentioned the shape of the articular surface furnished by the squamous bone for the lower jaw. This surface is transversely, not, as usually in Rodents, antero-posteriorly elongated; and it permits consequently of a much greater lateral movement of the jaw, correlated with which we find the molars above and below not with horizontal but with concave and alternately sloping grinding surfaces. The presence of six incisors in the upper jaw of young, and of four in that of adult Lagomorphi, is a third, the smaller size of the sockets for the lower incisors a fourth, point indicating less specialization in this suborder.
In the relatively small extent to which the temporal muscle is developed, in the great extent to which the lower jaw is developed behind the plane of its articular process, in the presence of a diastema between the anterior scalpriform teeth and the molar series, and in the keel-shaped presternum prolonged into the cervical region as a 'proosteon/ the Lagomorphi and most if not all other Rodents resemble many Ungulata, both Artio-dactyle (such as Sus) and Perissodactyle (Equns and Tapirus). In the reduction of the independence and importance of the fibula the Lagomorphi and the Myomorphi resemble each other and many or most Ungulata, and differ from all other Rodents, with some apparent exceptions, e. g. Pteromys and Castor. In the length and slenderness of a process given off by the squamosal posteriorly to the articular surface furnished by it to the lower jaw, which process not being anchylosed, as in many other mammals, to the tympano-periotic, nevertheless clamps it into fixed relations with the other skull-bones adjacent to it, the skulls of the Lagomorphi and many other Glires resemble those of some Perissodactyle Ungulata, whilst the presence of a third femoral condyle, and of an internal alisphenoid canal for the external carotid artery, are points in which they strikingly resemble all living Perissodactyla. On the other hand, a curious illustration of the combination in these Rodents of peculiarities which become separated in other divisions of the class Mammalia is furnished to us by the ischium of the Lagomorphi, which closely resembles the ischium both of the Ruminant and non-Ruminant Artiodactyles in what is considered to be a distinctive peculiarity of at least the latter of these two divisions of animals, viz. in the presence on the outer side of the bone a little way in front of its upper and posterior angle of a well-marked outstanding forwardly-curving process of bone.
The exposure in the dry skull of the turbinated bones in the nasal cavity by the deficient ossification of the lateral walls of that chamber is another point on which weight may be laid as connecting the Lagomorphi with some at least of the true Ruminants, e. g. Capra and Cervus.
As points of degradation in the Lagomorphi as compared with higher mammals we may note in the Rabbit and Hare the absence or great retardation of any anchylosis to each other of the basicranial bones, the sutures between the basioccipital and basisphenoid and between the basi-sphenoid and presphenoid remaining open not only when the occipital and interparietal bones are fused, but even after these bones have become abundantly fenestrated by senile absorption 1; the vertical and transverse perforations in the basisphenoid communicating with the pituitary fossa; the small antero-posterior length of the palatal plates of the palatine and maxillary bones leaving the stalked leaf-shaped end of the vomer exposed behind them, and the anterior end of the same bone exposed in front of them, when the dry skull is looked at along its base-line; the development of the 'foramina incisiva' into wide fissures continuous with the latter of the two sets of vacuities just spoken of; the persistence of open fontanelles in the occipital bone, in the interspace between that bone, the squamous, and the tympano-periotic, in the space, that is, which corresponds to the 'asterion' of Professor Broca, and in the interspace between the two last-named bones below the backwardly-running bar of the squamous; and probably also the singular fenestration or vacuolation of the anterior and upper part of the maxillaries.