by the projection of the maxillary ; ne vertheless, it is interposed between that bone and the sphenoid, and at the posterior extremity of the alveolar arch.
In the Guinea-pigs (Ancema, Fred. Cuv. ; Cavia,Ilig.; Musporcellus, Lin.), the lachrymal is large ; but it does not entirely form the upper [root of the pre-orbital ring, and the maxillary is not interrupted at this point. The pre i lymph wiriPr thnn it is hirrh We may, remark in this genus both the groove of the paca and the fossa of the agouti ; but the latter is situated higher up, as in the rats. The ascending branch of the maxillary is long and narrow. The bones of the nose are broader in front than behind. The jugal only commences towards the middle of the zygomatic arch ; the palatine, which superiorly does not penetrate either into the orbit or into the temporal region, extends in the palate only as far forward as the interval between the second and the third molar teeth.
In the kerodons, a small point of the frontal insinuates itself above, between the bones of the nose and the intermaxillaries, the ascend ing branch of the latter being very long, and extremely narrow at its origin, in the Bra zilian species. In this species, likewise, the pre-orbital ring is oval, and much elongated transversely, but formed entirely in the maxil lary bone as it is in the Guinea-pig ; whilst in the kerodon of Patagonia the lachrymal forms by itself nearly the whole vertical portion of its arch, so that the lachrymal is necessarily of very great size. Posteriorly, the maxillary touches by its apex a long point derived from the temporal external to the palatine ; the latter, however, is enclosed between the sphe noid and the maxillary, and mounts up into the floor of the orbit, when it is connected with the lachrymal bone. In the palate it is very deeply notched.
In the capybara (Hydrochccrus, Erxleben), the jugal is still shorter than in t'ne Guinea pig. The lachrymal is largely developed at the root of the vertical arch of the pre-orbital ring, but does not assist in forming it. The bones of the nose are very large and rectan gular. The ascending ramus of the inter maxillary, on the contrary, is extremely narrow, and is only united by its point to a point de rived from the frontal. The inferior hori
zontal arch of the ring is broad and flattened, with a little fossa at its base, as in the kero dons ; the maxillary is connected behind with the temporal, near the glenoid facet, external to the palate bone ; but what distinguishes capybara from them is, that this articulation is much more extensive, and that we cannot see, within, the long pterygoid apophysis and that portion of the palatine alluded to above. The external pterygoid alm are obliterated ; the internal alm terminate by a rounded plate, which is very far from reaching as far as the tympanum. The palatine encroaches upon the palate as far forward as the third molar tooth, and is interposed behind, between the maxillary and the sphenoid.
ln the viscache and the chinchilla, the bones of the nose are oval and elongated ; the ascending branches of the intermaxillaries very narrow at their origin ; but they enlarge as they approach the frontal, as in the jerboas. The maxillary, in both, forms the entire pre orbital ring; but in the viscache the vertical portion of the arch is doubled posteriorly, as m the helamys, by an ascending branch of the jugal, by the lachrymal, and by the frontal bones. At the bottom 'of the ring there is a deep groove, almost entirely separated from it by a vertical plate, as in the alactaga. In the chinchilla, the jugal does not reach as far as the lachrymal,and in the pre-orbital ring there is only a very superficial furrow, with no ver tical plate of separation. In both species the palatine is very deeply notched, it articulates with the maxillary, except externally, where a point of the posterior sphenoid touches the latter bone : moreover, on account of the en tire absence of the external wall of the ptery goid fossa, the palatine is found to occupy a considerable space in the floor of the orbit, be tween the orbital alm and the maxillary : it does not, however, mount upwards, as in the kerodon, between the latter bone and the frontal, to become connected with the lachry mal. The internal pterygoid apophysis is largely connected with the tympanum.