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Show I. ' 272 EXPLORATION Ol!' THE CANONS < JF THE OOLOHADO. alv 'Olur pint dip~ down between 1he front teeth. The maxillary ends anteriorly in the curve just described ; its o1 her boundaries are obscured in arlult; life. rrhe ide is nat; it suddenly rises in a broacl, thin, zygomatic plate, flush above with the general level of the top of the skull, there abutting (as shown by a long persistent suture) both with frontal and intermaxillary. This plate stands away nearly nt a right angle witiJ the axis of the skull, but very oblique to the other two planes. It circumscribes the orbit anteriorly; is excavated in the lachrymal region; its upper border is wirlened to a sharp-edged surface, and lopes gently outward, downwarrl, and backward; its thin under margin rises ton arly meet the upper, fini~hing the laminar portion, and continuing to the malar bone as an angular proccRs. A lachrymal bone is plainly indicated at the upper back part of the plate, but its extent and relations are not appreciable. The frontal is much contracted, especially across the middle, havi~g a somewhat hourglass-like superior outline, though both ends are angular. In front, it send8 a rectangular median process abutting against the nasals, and inclo ed between the intermaxillaries, and an acute lateral process on each side, entering a recess between intermaxillary and maxillary. These sutures seem per istent. Behind, the fronto-parietal and fronto-squamosal sutures are commonly obliterated; when appreciable, the bone is seen to unite with the extremely narrow parietals by a directly transverse straight line, and with the squamosals by an oblique line on each side. These sutures persist longer on top of the head than in the orbital region. The malar bone is a mere to:pliut, reduced coincidently with the great extension of the zygomatic spurs of both ~quamosal and maxillary. It it~ somewhat clubbecl anteriorly and overrides its ~upport; behind, it is itself overlapped.* The parietals, as already hinted, arc siugularly reduced in this family. In the skull of an old Geomys, the squamo-parictal suture is obscure or inappreciable, and the squamosals appear to meet each other a.t the abovedescribed ridge on the median line; careful ill pection, however, u ually reveals a very irregular and much overlapping squamo-parictal suture, defining • Alt!J~u~h tho zygo.rutL in t~i~ family i1:1 u. good 11tout nrcb, thi~:~ rctlucLiou of tho JUCilar 111.0pa.ro11 us for the dehca.to thrend-hko coud1t.10n of tho parts in tho next family, SftCCOIIIJ'idw. OOU:ES ON GEOMYS AND TBOMOMYS-OSTEOLOGY. 273 t.he parietals externally. These are of indeterminate shape, but tend to be narrowly rectangular; and, in Tlwmomys, a pair of pretty regular linear parietals is usually evident. There is constantly an interparietal-Sf)Uarish or pentagonal in Thomomys, rather triangular in Geomys. 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 Saccomyidre, 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 u ually fusing with the petrosal, is immensely developed, its superficies lying mostly in, and representing about half of each side of, the occipital Rmface. 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. rrhc petrosal does not share this unusual development, the bullre ossere 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 I'Jieatus, set quite free from its surroundings in a deep recess of the squamosal. The petrosal likewi. e is fissured away from the squamosal, but, in ndult 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 bcl?ind. The supra· occipital is squarish, with rounded corners; the cx-occipitals develop into moderate obtuse processes. Nearly all of the foramen magnum is vertical; the condyles arc rather small, and widely divergent superiorly rrhe 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 t.he glenoid fos a; the alisphenoid barely misses takmg a part in the mandibular articulation (us in some mar .. upials) ; 35 COL |
