In the jerboas (Dipus, Gmel.), the lines of separation between the frontal and parietal bones form a perfect cross; the inter-parietal bone is large and of a rhomboidal shape.
In the alactaga Plus jaculus, Lin.), a spe cies of the same genus, the inter-parietal is separated from the temporal by a broad cess divided from the occipital, which runs to join the parietal, as in the gerbilles (Ger Desmar, Illeriones,llig). The os petro sum occupies a considerable space in the occipital region ; but in the jerboa (Dipus) the great development of the ear renders impor tant changes in the structure of this portion of the skull indispensable. All the hinder por tion of the temporal bone is reduced to a thin osseous band, which is closely connected with the dilated tympanum and with the os petro sum, surrounding entirely the auditory canal. Another narrow band is derived from the sum mit of the occipital bone, which runs to be come united at a right angle with the above process derived from the temporal, so that a small triangular space is formed between the parietal, the occipital, and the temporal, in which is visible, at the upper part of the skull, that great vesicular mass, which occupies a part of its base and its posterior aspect. The paramastoid apophysis is a little tubercle which leans against the tl mpanum, and bounds posteriorly the articulating surface of the lower jaw.
In the helanws (Cape jerboa, or jumping hare), the structure of the skull in the vi cinity of the ear is analogous to that of Dipus. The petrous bones arise to the upper part of the cranium, and there oc cupy a considerable space between the tem poral and the inter-parietal bones, so that the temporals only give off a narrow band posteriorly, which does not reach the occipital bone, and does not surround the auditory passage, as in the jerboa. From the absence of any slip derived from the occipital bone, the upper portion of the os petrosum is not divided into two parts, as it is in the jerboa. The tympanum also is much less developed, and in its vicinity there is a very distinct paramastoid process. The inter-parietal, which is triangu lar, moreover, touches the parietals, the ossa petrosa, and the occipital. The lines of sepa ration between the frontals and the parietals form a cross ; the former are much larger than the latter. The anterior sphenoid is perfo•
rated at the bottom of the orbit. The tem poral alee do not ascend higher than the orbital, and remain widely separated from the parietal.
In the echinws (or porcupine rat of Az zara), the line which separates the frontal from the parietal bones is straight. The inter parietal is obliterated at an early age. A very distinctive character peculiar to the echimys is, that the occipital bone, as it descendslate rally towards the ear, bifurcates in such a way as to enclose the ascending portions of the tympanic bone and of the os petrosum, form ing by itself both the mastoid tubercles instead of the posterior one only, as is usually the case.
The anterior sphenoid gives off an orbital plate, which is moderately elongated ; but the posterior is almost excluded, both from the temporal region and from the orbit, owing to the length of the temporal front of the suture in this part. It is hardly visible except at the base of the cranium. The articulating surface for the lowerjaw is of a transverse form with out any marginal boundary behind.
In the capronws the bifurcation of the oc cipital bone is equally distinct, but its two processes join inferiorly in such a way, that only a small hole is left occupied by the os pe trosum. The orbital wing of the sphenoid is also less extensive.
In the porcupines the frontal bones are very wide in front between the lachryrnals. In young animals, a large semi-oval inter-parietal is met with ; but this bone, as well as the pa rietals and the frontals, unite at a very early period into one piece ; they also at an early age become consolidated with the ossa nasi, so that these seven bones not only- form one piece, but even become united to the tempo rals and to the occipital long before the bones of the face are anchylosed with each other. The os petrosum is scarcely discoverable at the back of the cranium, where it only forms a small tubercle embraced by two processes of the occipital, the interior of which represents the mastoid proeess of the temporal bone, and forms, external to the conclyles of the lower jaw, a broad paramastoid apophy sis. The pos terior sphenoid does not reach so far as the orbit, or rise above the anterior, which latter is but slightly visible upon the exterior of the skull.