RODENTIA. 



371 



post-orbital apophysis of the frontal, which 

 latter does not exist. The frontal bones, 



Fig. 253. 



Skull of the Ondatra. 



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. 



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 rents 

 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 remains 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 (Cricetus) 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 alas 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. 



Fig. 254. 



Skull of the Spalax typhlus. 



In the rat moles (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 

 paramastoid tubercle. 



In the jerboas (Dipus, Gmel.), the lines of 

 separation between the frontal and parietal 

 B B 2 



