RODENTIA. 



373 



bones and the inter-parietal, which is large, and 

 of a semi-oval shape, are at an early period 

 consolidated into one piece. The occipital 

 bone extends beyond the occipital crest in the 

 upper region of the skull, but the sides are 

 formed by the temporal. The petrous bone, 

 which is in early age consolidated with the 

 tympanic, is slightly visible by a narrow slip in 

 the occipital region. 



The tympana are much arched, but the 

 pterygoid processes do not touch them, because 

 .the foramen lacerum anterius, which is very 

 large, separates them. The superior maxillary 

 bone is articulated posteriorly with the pos- 

 terior sphenoid above the palatine, upon the 

 occipital region of the cranium. 



In the couia (Myopotamus, Commerson) the 

 sutures between the frontal and parietal bones 

 form a complete cross. The inter-parietal is 

 united to the surrounding bones at an early 

 age, but in young individuals it is very large, 

 and divided into two pieces j in the adult ani- 

 mal the zygomatic processes of the temporal 

 bone formed at their extremities a strong 

 hooked process, which winds down beneath 

 the jugal bone. The posterior sphenoid does 

 not enter into the composition of the orbit ; 

 the os petrosum is visible externally in the 

 occipital region of the skull, situated between 

 the two mastoid processes, which are both 

 formed by the occipital bone, but are of very 

 unequal length ; the external is pointed, the 

 inferior and internal is of much greater size, 

 running backwards and outwards, compressed, 

 pointed, and recurved. 



In the agouti, the frontal and nasal bones 

 remain separate, although the parietal and in- 

 ter-parietal are united into one piece ; in 

 young subjects the inter-parietal is of great 

 size, and semicircular in its shape. The or- 

 bital plate of the sphenoid enters largely into 

 the composition of the orbit, where it articu- 

 lates by it posteriorly with the temporal. In 

 the preceding genera it is to be remarked, that 

 the posterior sphenoid is joined to the frontal, 

 which is interposed between the temporal and 

 the orbital alae of the sphenoid; the tympana 

 regularly arched. The os petrosum does not 

 appear externally, but in addition there here 

 re-appears a small portion of the ethmoid, in- 

 terposed between the orbital ala of the sphe- 

 noid, the frontal and the lachrymal bones. 



In the capybara the hinder portion of the 

 cranium, as well as the occipital bone and the 

 inferior region of the temple, resemble what 

 is met with in the kerodon of Patagonia. The 

 paramastoid apophysis is excessively long, the 

 tympana are small. The petrous bone does 

 not appear at all in the occipital region of the 

 cranium. The parietals and inter-parietals are 

 consolidated into one piece at a very early 

 age, and separate, by a process more acute 

 than in any of the preceding genera, the cra- 

 nial portion of the temporal bone into two 

 branches ; the frontals are likewise united 

 together in very young animals. 



In the viscache the squamous portion of the 

 temporal bone is likewise deeply indented by a 

 point derived from the parietal. The posterior 



branch of this bifurcation, which is narrow at 

 its commencement, enlarges as it approaches 

 the occipital ridge. The inter-parietal and 

 the parietals are united into one piece, the 

 frontals are distinct, and the coronal suture is 

 transverse. The zygomatic process of the 

 temporal is directed almost horizontally, and 

 this bone remains widely separate from the 

 maxillary ; the posterior sphenoid unites with 

 this latter bone, external to the palatine, 

 which does not penetrate into the temple or 

 into the orbit : the posterior sphenoid has 

 no temporal ala, so that it reaches neither the 

 frontal nor the parietal bone a circumstance 

 which has been already remarked in preceding 

 genera. 



In the Jcerodons, the frontal bones remain se- 

 parate after the parietal and inter-parietal are 

 conjoined. The fronto-parietal suture is trans- 

 verse. The superior margin of the occipital is 

 bent upon itself at a right angle, as in the hares, 

 and articulates at the side of the cranium with 

 the temporals, terminating laterally by a long, 

 slender, vertical, paramastoid process. The 

 temporal gives off' posteriorly a lamina, or 

 apophysis, which descends more or less in 

 different species between the tympanum and 

 the petrous bone. The latter bone is not 

 visible externally in the occipital region, but 

 is apparent upon the side of the head, above 

 and behind the auditory passage. The con- 

 nections of the bones in the orbit are not less 

 remarkable than in the Guinea-pig. The tem- 

 poral is in like manner developed at the ex- 

 pense of the posterior sphenoid ; but it is the 

 former which becomes united by its apex to 

 the extremity of the maxillary bone, the 

 sphenoid which runs parallel with it only ap- 

 proaching the maxillary, from which it is 

 separated by a slip derived from the os palati. 

 The temporal, as in the preceding genera, is 

 united in the orbit to the anterior wing of the 

 sphenoid in the Guinea-pig, with this differ- 

 ence, however, that the temporal leaves it 

 free externally. The petrous bone occupies 

 a considerable surface in the occipital region 

 of the skull, and likewise furnishes a mastoid 

 tubercle at the base of the paramastoid 

 apophysis, which resembles that of the couia; 

 and which, at first, running outwards and 

 backwards, suddenly bends inwards and for- 

 wards. The petrous bone occupies a large 

 part of the occipital region, where it presents 

 a flattened surface ; it also furnishes a mastoid 

 tubercle at the base of the paramastoid 

 apophysis, which resembles that of the couia, 

 and which, at first, directed outwards and 

 backwards, afterwards suddenly bends in- 

 wards and forwards. 



In the chinchilla the connection of the 

 frontal and of the parietal bones, as well as 

 those of the sphenoid with the maxillary and 

 with the temporal, are the same as in the 

 viscache, but the great development of the 

 ear causes differences in the posterior region. 

 The paramastoid apophysis, which is strongly 

 marked, is closely applied against the tympa- 

 num, and does not project inferiorly. The 

 petrous bone, instead of presenting a flat sur- 

 B B 3 



