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Show COUES ON GEOMYS AND THOMOMYS-OSTEOLOGY. 273 the parietals externally. These are of indeterminate shape, but tend to be narrowly rectangular; and, in Thomomys, a pair of pretty regular linear parietals is usually evident. There is constantly an interparietal-squarish or pentagonal in Thomomys, rather triangular in Oeomys. With such state of the parietals, there is a corresponding overdevelopment of the temporal bone, especially of its squamosal element, though not to the extraordinary extent witnessed in Saccomyidce, where the whole bone is blown up like a bladder. The squamosal roofs over most of the cranial cavity, and alone forms (with the exception of a little place occupied by the interparietal) the whole occipital or lambdoidal crest. The mastoid, which persists distinct from both squamosal and occipital, though usually fusing with the petrosal, is immensely developed, its superficies lying mostly in, and representing about half of each side of, the occipital surface. It develops a moderate "mastoid process'', lying against the postero-external corner of the squamosal, and looking like a duplicate of the paroccipital process that lies against its opposite extremity. The petrosal does not share this unusual development, the bullse ossese being, in fact, smaller than they are in Arvicola, for instance; they swell but little below the baso-occipital plane. The tympanic develops into a tubular naeatus, set quite free from its surroundings in a deep recess of the squamosal. The petrosal likewise is fissured away from the squamosal, but, in adult life, the tympanic, petrosal, and mastoid are consolidated. The upper and lower parts of the occipital bone are at right angles with each other; the basi-occipital is horizontal upon the floor of the skull, while the superior and lateral elements are perpendicular behind. The supra-occipital is squarish, with rounded corners; the ex-occipitals develop into moderate obtuse processes. Nearly all of the foramen magnum is vertical; the condyles are rather small, and widely divergent superiorly The suture with the basi-occipital, which persists for some time, is ordinarily the most conspicuous of the sphenoidal relations which may be appreciated in examination of adult skulls. Close inspection, however, shows the squamo-sphenoid suture just inside the glenoid fossa; the alisphenoid barely misses taking-a part in the mandibular articulation (as in some marsupials) ; 35 COL |
