| OCR Text |
Show COUES ON GEOMYS AND THOMOMYS-OSTEOLOGY. 271 less preparation of the skull, and in such a way that scarcely a suggestion of their former presence is left. Behind the pterygoids, the conspicuous bullae ossae appear convergent anteriorly to touch the former, prolonged into a tube exteriorly. Between them, the basi-occipital space is cuneiform (especially in Thomomys-more nearly quadrangular in Geomys), with a median ridge and lateral depressions, nicked behind by a small portion of the foramen magnum. The skull finishes behind by an irregular curve, substantially the same as that described in speaking of the occipital plane. In all but the oldest animals, the following sutures, or, at any rate, traces of them, persist: internasal, naso-intermaxillary, maxillo-intermaxillary, fronto-nasal, fronto-intermaxillary, and fronto-maxillary; maxillo-malar, squamo-parietal, squamo-malar, squamo-mastoid, occipito-mastoid, occipito-petrosal; basi-occipito-sphenoid; and there is fissured separation of the petrosal and tympanic from the squamosal. The various intricate relations of the palatals, and of the ''sphenoid" as a whole, are inappreciable in the adult skull. Detailed relations of such of the individual bones as can be made out from the material before me here follow: The nasals reach back to a point opposite the anterior root of the zygoma, but extend little, if any, in the other direction, beyond the intermaxillaries. For two-thirds their extent they are narrow and approximately parallel in the examples of Geomys before me, and then rapidly expand. In all the Thomomys I have seen, they widen regularly from the base to tip. They are flat at first, but toward the end become somewhat volute or scroll-like. They remain permanently distinct from the intermaxillaries, and have failed in no case to show me separation from each other. The intermaxillaries run up on the forehead farther than the nasals-to or beyond the back instead of front border of the zygomata, being received in a deep ernargination of the frontal. Below, similarly, they run far down on the false palate, ending opposite the back end of the incisive foramina. Their course around the side of the rostrum (maxillo-intermaxillary suture) may usually be traced as a strongly convex curve between the upper and lower points just mentioned, the most forward portion of the curve lying nearly midway between zygoma and incisors. The lateral surface is thrown into a curved elevation, denoting the track of the incisor within. A strong |
