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Show 274 EXPLORATION OF THE CANONS OF THE COLORADO. the orbito-sphenoid, lining the orbit behind, rises nearly to the top of the skull. The mandible remains for consideration. This is eminently characterized by its massiveness and the emphasis of its various ridges and angles. Nevertheless, the symphysis, though extensive, is incomplete. Instead of an edge below, the bone presents a broad, smooth, flattened area, bounded on the sides by a ridge indicating the limit of masseteric muscular attachment. The angle of the jaw is strongly exflected in a peculiar way. An oblique plate (the "descending process" in many rodents) arises from the inner side of the body of the bone, and curves strongly backward and outward, ending far exterior to the main part of the bone as a strong laminar process. Just •nside of this, between it and the condyle, there is a strongly-marked, smooth, upright protuberance. This is where the root of the incisor pushes up from the inside. To the inner side of this knob, again, rises a third protuberance; it is the condyle, rather small, and of no noteworthy features. (It appears particularly small when compared with the glenoid cavity, which, as I should have remarked before, is of unusual width.) Thus the mandibles, viewed from behind, present the curious appearance of three prongs-condyle, incisor-knob, and exterior process. The appearance of trifurcation is best marked in ThomomySj where the tooth-knob is most prominent, and separated by deepest notches from the processes between which it stands. In addition to -» all these prominences, a slender, falcate, acute coronoid rises in front, and overtops the rest, being separated from the condylar ramus by a deep notch. There is a deep excavation between the thin laminar basis of the coronoid and the molar alveolus. The foramen of the inferior maxillary nerve appears on the inner side of the root of the condylar ramus. The dental formula has been already given. The molar dentition appears weak and slight in comparison with the enormous incisors. The under incisors, as already said in effect, run the whole length of the jaw, and push up a knob of bone behind. They are of the ordinary scalpriform construction, quite flat-faced, with converging sides, and beveled to an edge behind. The superior incisors describe nearly a semicircle through the intermaxillaries, and far into the maxillaries, to below the root of the zygoma. They are |
