The inter-parietal remains for a long time distinct, it is very large, and is situated be tween the two parietals, the two temporals, and the sphenoid.
The posterior sphenoid mounts much higher into the temporal region than in the genus Bathiergus, and joins both the tem poral and the frontal. The parietal does not reach within a considerable distance of it. The tympanum is prominent, and rests posteriorly upon a well-marked paramas toid process. The suture between the tympanic and the petrous bone exists till a late period. The occipital portion of the petrous bone forms no tubercle, but pene trates deeply into the occipital.
In the rats, properly so called, the frontals which remain separate for a long period, are distinguished from the parietal by the inter vention of a straight transverse line. Their inter-parietal is rectangular and placed trans versely, but does not reach as far as the temporal bones. The posterior sphenoid does not mount into the temporal region to a greater height than the anterior; it there joins the frontal, but reniains separated by a con-. siderable space from the parietal.
In the gerbilles the fronto-parietal suture forms the arc of a circle. The inter-parietal is broad transversely ; its suture with the parietals is nearly straight, and it is embraced posteriorly and laterally by the occipital. The temporal, upon the sides of the cranium, is comparatively small in front ; it touches the frontal at the extremity of the frontop-arietal suture ; posteriorly it continues the suture, which, descending from the inter-parietal angle, separates the parietal from the occipital : the latter bone is deeply notched to receive the os petrosum, which it separates from the inter parietal by a quadrilateral process. The ar rangement of the bones in the orbit resembles that of the genus Mus. The tympana are extremely vesicular and prominent ; they bound posteriorly the glenoid cavity, which resembles a deep furrow. There are small paramastoid apophyses closely applied to them.
In the hamster (Crieetus) the inter-parieta is a small tri angular bone ; the temporal is extended at the expense of the parietal, and stretches as far back as the occipital. The
orbital and temporal alw of the sphenoid are arranged in the orbit as in the rats. There is no paramastoid process behind the condyles of the lower jaw.
The same observations are applicable to the dormice, but their inter-parietal bone is elongated transversely, so as to touch both the occipital, the parietal, and the temporal ; the posterior sphenoid, moreover, only touches the maxillary by its apex. A little process, derived from the palatine, separates them below. These animals have the zygomatic arch situated lower down and broader than the hamsters. Their tympana are much larger, well rounded, and in contact with the inter nal pterygoid processes.
In the rat nzoles (Spalax) the occipita bone is flanked by the ossa petrosa and the temporals, to form the occipital surface of the cranium ; but the occipital suture is as usual situated in front of the occipital crest— a circumstance which encroaches much upon the parietal bones. This disposition is in relation with the strength of the muscles that support the head. The parietal encroach upon the frontal by a pointed process. The temporal ridges unite together to form a single sagittal crest, and the zygomatic arches are very prominent, externally corresponding to the great size of the temporal muscles. There is no inter-parietal bone. The tympana are but slightly arched.
In the rhyzomys of Sumatra, on the con trary, it is the frontal which extends by a pointed process between the parietal ; and, moreover, the temporal bones mount upwards very high upon the cranium, so as to join the frontal; there is no inter-parietal bone visible. The os petrosum is visible upon the occipital aspect of the cranium. A process derived from the temporal, which contributes to form the occipital ridge, is interposed between the os petrosum and the external auditory tube. The tympanum is lofty and well-rounded, and separated from the petrous bone behind by a process of the occipital, which terminates in a pararnastoid tubercle.