In the marmot, the frontal and the parietal bones are at a very early age consolidated into a single piece, and an inter-parietal bone is not discoverable even in very young mar mots. The frontal bones, which are extensively penetrated by the two ossa nasi, penetrate deeply between the parietals, which latter are narrow, and the sutures which connect them to the temporal remarkably straight and pa rallel. The occipital suture is situated a little in front of the occipital crest, with which it runs nearly parallel.
One-third of each side of this crest is formed by the petrous bone, which infringes slightly upon the occipital surface of the cranium. External to the tympanum, and a little behind it, there is a mastoid process ; behind which is another (the paramastoid), formed by the oc cipital bone. The tympanic bones are round and much inflated ; they are consolidated at an early age with the petrous bones. In the temple the posterior sphenoid mounts consi derably upwards, but nevertheless only joins the temporal and the frontal, the parietal not descending sufficiently low. The orbital ala of the sphenoid enters but little into the com position of the orbit.
In the squirrel the separation between the parietals and the frontal bones is likewise obliter ated at a very early period. The inter-parietal also becomes soon confounded with the parietal ; but in very young subjects its presence is well marked ; it is of semi-circular form. There is, moreover, a special point of ossification in the centre of the cross, formed by the frontal and parietal bones. The glenoid cavity is more deeply excavated than in the marmot.
In the beaver, the frontals are consolidated together at a very early age ; the parietals also unite to each other and to the frontals, even before the inter-panetal has become blended with them. The inter-parietal is triangular, and in very young subjects is double. The glenoid cavity is bioader than it is long ; its external border only is formed by the jugal bone ; its posterior margin is al together free. The tympanum is altogether formed by the tympanic bone. Between the two tympana the basilar region of the cranium is hollowed to such an extent as to be partly membranous, even in very old animals.
There are two mastoid tubercles placed near to each other ; one formed by the petrous bone, the other by the occipital. The petrous bone becomes united at an early period to the tympanic bone, a pointed apophysis of the temporal insinuating itself between them, behind the external auditory foramen.
The posterior sphenoid joins to the frontal in the temporal region ; the anterior sphenoid then mounts up very high ; and in adult spe cimens, when the molar teeth have come down, and the maxillary bones are no longer distended, there is inferiorly a compressed portion, by which the sphenoid joins the maxillary and the palatine bones, and which forms a partition pierced with several holes between the bottom of the two oibits.
In the Cape mole (Bathiergus) the sutures at the upper part of the cranium are disposed much in the same way as in the beaver, only in the larger species the temporals are broader anteriorly, and encroach upon the frontal in front of the parietal. The inter-parietal is of an oval shape. The temporal presents, posterior to its arch, a large fissure, which is not closed by the os petrosuni : the latter bone, however, on the other hand, fills up a deep notch, which exists on the external border of the occipital bone. The paramastoicl apophysis is dilated into a prominent plate.
The anterior sphenoid, which enters but little into the composition of the orbit, forms beneath it a simple lamella, but which is not perforated. The posterior sphenoid does not ascend into the temporal, but a considerable prolongation of the frontal bone comes down to unite with it, about the level of the edge of the glenoid cavity ; it also furnished a process to be articulated both with the pala tine and the maxillary bones.
In the ondatra and the water voles the parietal bones are, as it were, imbedded in the shape of a disk between the temporals. The temporal, moreover, furnishes a prominent projection, that might be mistaken for the post-orbital apophysis of the frontal, which latter does not exist. The frontal bones, which are consolidated together long before the parietal, are much reduced in size, in con sequence of the extension of the temporals, and the narrowness of the inter-orbital space.