370 



RODENTIA. 



by a long process, between the ascending point 

 of the inter-maxillary bone and that portion 

 of the maxillary which forms the cheek : the 

 parietals remain for some time distinct from 

 each other, and from the inter-parietal ; which 

 latter, in the rabbit, is small, and resembles 

 an ellipse placed transversely : in the hare 

 this last bone can only be detected in very 

 young specimens, when it is found to con- 

 sist of two small pieces, which are separated 

 by a prominent angle of the occipital. The 

 petrous portion of the temporal bone occu- 

 pies a large triangular space in the occipital 

 region of the skull. The mastoid process is 

 entirely formed by the occipital bone; but the 

 os petrosum furnishes a parallel process, which 

 embraces the temporal externally, and at an 

 early period it becomes united therewith. 

 The tympanic portion of the temporal is con- 

 siderably arched, but is far from reaching the 

 pterygoid processes. The temporal alae of 

 the posterior sphenoid do not mount up very 

 high, and do not reach the frontal, from which 

 they are separated by the anterior sphenoid 

 and by the temporal, still less do they ap- 

 proximate the parietal bones, which do not 

 descend so low as the temporal. 



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 islikewiseobliter- 

 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-parietal has become 

 blended with them. The inter-parietal is 



triangular, and in very young subjects is 

 double. The glenoid cavity is broader 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 orbits. 



In the Cape mole (Bat/iiergus) 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 petrosum : the latter 

 bone, however, on the other hand, fills up a 

 deep notch, which exists on the external 

 border of the occipital bone. The paramastoid 

 apophysis is dilated into a prominent plate. 



Fig. 252. 



Skull of the Bathiergus maritimus. 



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 



