To these points, dependent upon a deficiency of ossification, may be added the involution of the angle of the lower jaw, which represents, though but rudimentarily, the inversion of that part of the jaw in the Marsupials; and the fusion of the optic foramina into a single mesial foramen bounded inferiorly by the presphenoid much as in Birds.
Other points worthy of note in the Rabbit's skeleton, either as compared with those of most Rodents of other suborders, or as compared with those of other mammals, are presented to us in the imperfect differentiation of the coronoid from the ascending ramus of the lower jaw; in the approach to horizontality in the symphysis of that bone; in the large size and backward direction of the tympanic process of the tympano-periotic; in the small size of the infra-orbital canal and of the anterior part of the malar process of the maxillary, and the large size of the free backwardly-project-ing process of the malar bone proper; in the presence of large supra-orbital processes attached in the middle and projecting freely at either end of their length; in the fixed attachment of the upper lamellae of the ethmoid to the nasal, constituting 'naso-turbinal' bones; the loose often lost attachment of the much convoluted inferior turbinals to the maxillaries 1; in the formation by the posteriorly expanded vomer of a floor to the true olfactory portion and of a roof to the lower narial or respiratory portion of the nasal cavity; in the wide interval between the pterygoids and the tympanic bulla, and in the presence in this last bone of a canalis caroticus.
The pitted appearance of the interparietal bone, of the upper part of the occipital bone in apposition or fused with the interparietal, and of the upper arch of the first cervical vertebra, is worthy of note as suggesting a comparison with Lophiomys, and with the rugose 2 zygomatic and frontal bones in the Paca (Coelogmys pacd). In aged specimens this pitting almost amounts to fenestration. The anchylosed tibia and fibula are specially noteworthy.
1 The persistence of patency in the sutures of the basis cranii appears to possess considerable morphological value, but this cannot be said of the sutures of the roof of the skull. For example, the interparietal anchyloses very early in the Subungulate Hystricomorphi, but it does the like also in Sciurus, whilst it remains distinct for a long while in Myoxus, Castor, and the Murini, as well as in the Lagomorphi.
The imperfection of the clavicles in the Rabbit and Hare prepares us for their entire absence as reported to exist in some few of the Subungulate Hystricomorphi, and contrasts with their complete development in some families of the same suborder (Chinchilloides, Spalacopodoides), as also in the entire suborders Myomorphi less Lophiomys, and Sciuromorphi, and even in the subgenus of the suborder Lagomorphi represented by the tail-less Hares, Lagomys. The presence of a backwardly and downwardly projecting process of the acromion is similarly a peculiarity observable in the Lagomorphi and certain Hystricomorphi, whilst it is absent in many Rodents, though present in other orders of Mammalia, and notably in the Elephant.
The non-development of fangs confers the same privilege of perpetual growth on the molars of the Lagomorphi, the true Cavies, and the Chinchillas, which is enjoyed by the incisors of all Rodents. The white colour and the shortness of the incisors again are points of similarity between the Hares and the Cavies. The number of molar teeth is greater in the Hare and Rabbit than in any other Rodents, being 6/5 as against 5/5 in the allied subfamily Lagomys, against 4/4 in Hystricomorphi and Sciuromorphi, 3/3 in Myomorphi, and 2/2 in Hydromys. The vacuity in the lower jaw, posteriorly to the socket for the last molar tooth, and the vertical upgrowths from the tubercles of the second to the eighth pair of ribs, are peculiarities in the skeleton of the Lagomorphi.
In the ossa ilii the glutaeal surfaces are much more extensive than the iliac, a line drawn forwards from the tubercle for the short head of the rectus just above the acetabulum along a faintly-marked and rounded ridge represents the acetabular border of other Rodents, such as the Beaver, and shows the limit of the respective surfaces. The incisura acetabuli through which the blood-vessels and nerves enter for the supply of the hip-joint is reduced in size, and the rim of the acetabulum is interrupted only by a linear fissure. The symphysis of the pubis is deep.
1 The maxillo-turbinals are more complexly and finely convoluted in the Rabbit than in the Hare, the subterranean habits of the former of these animals creating a greater need for warming the inspired air. Similarly, as remarked by Professor Flower (Osteology of Mammalia, p. 183, second edition, 1876), in the Elephant, where the inspired air is sufficiently warmed by having to pass along the elongated proboscis, the maxillo-turbinals are wholly aborted.
2 See Flower Osteology, p. 156; and Waterhouse, History of Mammalia, ii. p. 369, who suggests that periodical deposition analogous to that of the horns of deer causes this. Compare the strange account of Lepores cornuti given by Schreber, Saugethiere, i. Taf. cclxxxiii. B; Pallas, Novae species Glirium, p. 14, ibique citata.
The skull of the tame Rabbit differs from that of the wild in having the roof of its brain-containing portion much flatter as measured either from before backwards or from side to side than is the case in the very distinctly arched calvaria of the wild variety. The lateral boundaries of the same cavity as constituted by the squamous are much more wall-sided than in the wild race, and instead of curving gradually into a vaulted vertex they are defined or delimited off from it by largely-developed anteroposte-riorly running ridges. The height of the occipital foramen is less relatively to its breadth, its upper and lower borders not being emarginated into secondary curves as in the wild variety. The length of the entire skull is considerably greater relatively to its breadth, though not relatively to the size of the entire body. This may be double that of a wild specimen, whilst the absolute breadth of the skull may be identical in the two subjects of comparison, and the absolute length may be less than 30 per cent, greater in the tame than in the wild variety.
The lines and processes of the cranium and lower jaw are less sharply defined and sculptured than in the wild variety, and the surface of the cranial bones generally is inferior in gloss and polish. The same applies to the bones of the trunk and limbs in many domestic animals as compared with animals of the same species in a wild state, and indeed is usually more clearly appreciable than in the case of the two varieties here compared with each other.
For the possession of rootless molars by other Rodents (Octodon, Capromys, most Arvicolae); for that of molars with short roots or with roots incomplete or late to be developed, by the Agouti, by the Paca s. Spotted Cavy, by the Beaver and the Porcupine; for that of rooted molars by the true Mice and the Squirrels, see Owen, Odontography, p. 401; and by Leporidae in their milk dentition, see Hilgendorf, Monatsber. Ak. Wiss., Berlin, 1876, p. 673.
For the presence in Leporidae of a perfect investment instead of, as in all other living Rodents, merely an anteriorly placed plate of enamel on the incisors, see Hilgendorf, l. c. But preparations made by Mr. C. S. Tomes suggest that this perfect investment exists only in the enamel membrane of the developing tooth.
For numerous other points of similarity between the Hares and the Cavies, see Waterhouse, History of the Mammalia, ii. p. 156, 208; Buffon, cit. Pallas, l.c. p. 29.
For the Shoulder-Girdle of the Rodents, see Parker, Shoulder-Girdle, 1868, pp. 207-210; and for the mesial prolongation of the praesternum in Lepus and Cavia into the cervical region, see Plates xxiv and xxv; and for the same prolongation in Ungulata, see PI. xxix.l. c.
For a detailed comparison of the osteological differences between tame and wild Rabbits, see Darwin, History of Plants and Animals under Domestication, i. pp. 120-136, 2nd ed. 1875.