372 



RODENTIA. 



bones form a perfect cross; the inter-parietal 

 bone is large and of a rhomboidal shape. 



In the alactaga (Mus jacidus, Lin.), a spe- 

 cies of the same genus, the inter-parietal is 



Fig. 255. 



Skull of the Dipus hersipes. 



separated from the temporal by a broad pro- 

 cess divided from the occipital, which runs 

 to join the parietal, as in the gerbilles (Ger- 

 billus, Desmar, Merioncs, Ilig). The os petro- 

 sum occupies a considerable space in the 

 occipital region ; but in the jerboa (Dipm] 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 fromthe 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 apophvsis is a little tubercle 

 which leans against the tympanum, and bounds 

 posteriorly the articulating surface of the lower 

 jaw. 



In the helamt/s (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 alas do not ascend higher than the 

 orbital, and remain widely separated from the 

 parietal. 



In the echimys (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 lower jaw is of a transverse form with- 

 out any marginal boundary behind. 



In the capromys 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 lachrymals. 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 process of the temporal bone, and 

 forms, external to the condyles of the lower 

 jaw, a broad paramastoid apophysis. 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. 



In the cocndon the parietal bones are pro- 

 longed by a pointed process between the 

 frontals ; the suture between them, and also 

 between the inter-parietal and occipital, is ob- 

 literated. The tympanum is much arched ; the 

 os petrosum hardly appears in the occipital 

 region of the skull, but is slightly visible a little 

 behind the tympanum above the paramastoid 

 apophysis, which is of moderate size. 



In the paca the frontal bones are much 

 elongated ; the suture between them and the 

 parietals is transverse ; the temporal extends 

 backwards as far as the occipital ridge, and 

 descends behind the tympanum over the base 

 of the mastoid process, the point of which be- 

 longs to the occipital bone. The relations of 

 the sphenoid orbital plates are as in the 

 agouti, but the tympana are less prominent. 



In the fcetus, and in very young subjects, 

 there are two parietal and two inter-parietal 

 bones ; but these four pieces become at an 

 early age consolidated into one. 



In the Guinea-pig (Cavia) the parietal 



