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Show 274 EXPLOl~ATION OF 'l'BE OAN"ONS 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 character-ized by its massiveness and the emphasis of its various ridges and angles. Nevertheles~, the symphysis, though extensive, is incomplete. Instead of an edge below, the bone presents a broad, smooth, flattened urea, bounded on the sides by a ridge indicating the limit of masseteric muscular attachment. The angle of the jaw is strongly exAected in a peculiar way. An oblique pla1e (the "desccncling process" in many rodents) ari!'o;es from the inner side Jf the body of the bone, and curves strongly backward and outward, ending ~ar exterior to the main part of the bone as a strong laminar process. Just ' JJsidc of this, between it and the condyle, there is a strongly-marked, smooth, upright protuberance. This is where the root of the inci or 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. rrhe appearance of trifurcation is best marked iu Tltomomys, where the tooth-knob is most prominent, and separated by deepest notches from the processes between which it stands. In addition to all the e prominences, a slender, falcate, aeute coronoid rises in front, and overtops the rest, being separated from the condylar ramus by a deep notch. There is a deep exeavation between the thin laminar basis of the coronoid and the molar alve<?lus. The foramen of the iuferior maxillary nerve appears on the inner sicle 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 Aut-faced, with converging sides, and beveled to an edge behind. The SUJ)Cnor mct:s~or·s desct·t'be near lY a emi· c·l rc1 e tH} ough the intermaxillaric:s, and far into the maxillaries, to below the root of the zygoma. They are OOUES ON GEOMYS AND THOMOMYS-OSTEOLOGY. 275 of distinct character in the two genera, furnishing the most ready means of diagnosi~, not only of the genera, but of the species of Geomys, as already fully given in the body of this paper. The molars arc perennial rootless prisms, as in Arvicolince and many other hard gnawers, but are smal1 and of a very simple structure-at least in comparison with the complicate character which obtains in many rodents. The whole molar series is scarcely one-seventh of the length of the skull. They are implanted very obliquely to suit the peculiar conformation of the parts. The axis of the anterior upper molar slopes backward at an angle of about 45°, and the rest succeed with rcgubrly-diminishing obliquity. The relation is reversed in the lower jaw, where the back molar slopes forward, the rest becoming successively more nearly perpendicular. There is the same number of teeth in both jaws, and they are quite similar in construction. The anterior molar in each jaw is a double prism; the others are single and simple, elliptical in cross-section, the first being a pair of ellipses laid together like a short broad figure-of-eight, and the last approaching a cylindrical figure. The relation of the molars to each other is somewhat singular. Their roots are all widely diverging, but their crowns come into close contact. This is effected by the curve in -their axis. Thus the front upper molar is curved with the convexity posterior; the rest are curved successively more and more, with the convexity anterior. Similar characters mark the under molars, though less strongly; and there is seen in these teeth, especially in the anterior ones, a lateral as well as fore-and-aft curve. This shape appears to be forced upon the teeth by the peculiar conformation of the alveoli. 'rhe molars are quite similar in the two genera, and scarcely afford diagnostic characters, especially since there is some change in the details of the molar crowns with acre and wear of the teeth. On the whole, however, it may be observed that 0 in Geomys the molars-the immediate ones, at any rate-are more perfectly elliptical than they are in Thomomys, where a pinching-together of the extc rior portion of the ellipses tends to result in a pyriform contour. The principal cranial and dental characters of the two genera which compose the Geomyidce may be shortly contrasted, as follows: I |