In the coendou 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 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 fbranzen lacerunt 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 ; 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 alx 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 tbe 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 corona' suture is transverse. The zygornatic 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.