In the kerodons, the frontal bones remain se parate after the parietal and inter-parietal are conjoined. The fronto-parietal suture is trans verse. The superior margin of the occipital is bent upon itself at a right angle, as in the hares, and articulates at the side of' the cranium with the temporals, terminating laterally by a long,, slender, vertical, paramastoid process. The temporal gives off posteriorly a lamina, or apophysis, which descends more or less in different species between the ty mpanum and the petrous bone. The latter bone is not visible externally in the occipital region, but is apparent upon the side of the head, above and behind the auditory passage. The con nections of the bones in the orbit are not less remarkable than in the Guinea-pig. The tem poral is in like manner developed at the ex pense of the posterior sphenoid ; but it is the former which becomes united by its apex to the extremity of the maxillary bone, the sphenoid which runs parallel with it only ap proaching the maxillary, from which it is separated by a slip derived from the os paktti. The temporal, as in the preceding genera, is united in the orbit to the anterior wing of the sphenoid in the Guinea-pig, with this differ ence, however, that the temporal leaves it free externally. The petrous bone occupies a considerable surface in the occipital region of the skull, and likewise furnishes a mastoid tubercle at the base of the paramastoid apophysis, which resembles that of the couia; and which, at first, running outwards and backwards, suddenly bends inwards and for wards. The petrous bone occupies a large part of the occipital region, where it presents a flattened surface; it also furnishes a mastoid tubercle at the base of the paramastoid apophysis, which resembles that of the couia, and which, at first, directed outwards and backwards, afterwards suddenly bends in wards and forwards.
In the chinchilla the connection of the frontal and of the parietal bones, as well as those of the sphenoid with the maxillary and with the temporal, are the same as in the viscache, but the great development of the ear causes differences in the posterior region. The paramastoid apophysis, which is strongly marked, is closely applied against the tympa num, and does not project inferiorly. The petrous bone, instead of presenting a flat sur face in the occipital region of the skull, is ex tremely dilated, insomuch, indeed, that this dilatation appears in the upper wall of the skull, in the shape of two large projections, bounded in front by the parietals, internally by a plate common to them and the occipital bone, and posteriorly by a long narrow trans verse projection from the occipital, which is in contact with the petrous bone, and exter nally by another thin and pointed slip, which forms the posterior termination of the tempo ral bone, and which projects above the audi tory meatus to join that derived from the occipital. We have seen, above, that in the jerboa a siinilar disposition exists.
Bones of the face. —In the Rodentia the intermaxillary bones are of enormous dimen sions, on account of the great size of the inci sor teeth, so that the maxillary bones are pushed very far backwards ; these latter form a large portion of the inner wall of the orbit, into the composition of which the os palati enters but slightly, and sometimes, indeed, not at all. The anterior boundary of the orbit is formed by a process of the maxillary bone, which proceeds to contribute to the formation of the zygomatic arch in such a way that the os malx is, as it were, suspended in the centre of the arch between the apophy sis, derived from the maxillary and the zygo matic process of the temporal bone.* It joins neither the frontal nor the sphenoid. The elongation of the ossa nasi is such that the opening of the nose is situated quite at the extremity of the snout.
In the aye-aye the bones of the nose are short and broad. The intermaxillaries mount up along their sides by a broad process, which occupies part or the snout, and are articulated to the frontal by a tolerably broad space ; they touch, likewise, the lachrymals which encroach upon the cheek ; while the canal situated between the latter bones, the maxil lary, and the jugal, is out of the orbit. The jugal apophysis of the maxillary arises oppo site the second molar tooth, and the boundary of the jugal bone is placed at the anterior base of the zygornatic arch. It articulates with the lachrymal, both within and without. The orbit is very broad, and furnishes a large post orbital apophysis, which joins that derived from the frontal bone. The palatine bone advances but a little way into the palate, ter minating by a straight suture between the last molar teeth. The palatine portion of the pterygoid aim is simple ; their sphenoidal portion is divided into two laminae, the ex ternal of, which is prolonged as far as the tympanum, tO which it is articulated, as well as to the inner border of the glenoid surface. In the temporal region, the palatine bone re mains behind the posterior margin of the maxillary, between the latter bone and the two sphenoids, only touching the frontal by its apex.
In the hares, the intermaxillary bone pre sents, besides its palatine portion, which is large, a long ascending apophysis, which is at first imbedded between the maxillary and the os nasi, and subsequently between the latter and the apophysis of the frontal, to which latter it is connected. All that portion of the maxillary bone which forms the cheek is, in the adult animal, riddled with holes, so as to have the appearance of lace-work. The la chrymal in the orbit is tolerably large ; exter nally, it gives off a blunt hook, beneath which is the lachrymal canal, situated upon the very edge of the orbit. The zygomatic portion of the maxillary bone is short; its inferior margin forms a ridge, which projects slightly externally, and presents a flattened surface, from which arises one of the portions of the masseter muscles. It is this surface which we shall see in other ..Rodentia become rounded into a more or less oblique vaulted space, and in others become transformed into a wide ring. The union between the maxil lary' and the jugal bones is so soon obliterated, that unless we examine very young indivi duals we should be tempted to believe that no jugal existed. This latter bone is arched in feriorly, and extends by means of a process beneath the zygomatic portion of the temporal bone. Besides the floor with which it covers the roots of the teeth, the maxillary gives off a narrow plate, which mounts into the orbit as high as the os frontis, between the lachry mal, from which, however, it is separated by a membranous space and the anterior sphenoid. The vomer is visible at the hinder part of the septum, which separates the foramina inci siva. The palatine occupies beneath the an terior sphenoid in the orbit a much greater space than in other Rodents ; inferiorly it extends as far as the third molar tooth, and is deeply indented as far as the fourth. The ptery goid ake extend to the azygos portion, or to the body of the anterior sphenoid, but they are separated from that of the posterior sphenoid by a membranous space on each side. The posterior sphenoid has on each side two pterygoid plates, which are both of them con tiguous to those of the palate bones; the in ternal ones terminate in a slender point or style. • In the lagomys, the base of the zygomatic arch gives off' a process, which is directed downwards ; and the jugal bone, after having passed beyond the zygornatic process of the temporal, is prolonged directly backwards into a lengthened point.