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Show 278 EXPLORATION OF TOE CAN"ONS OF THE COLORADO. vertical plane of incisors; rostrum broad, parallel-sided. Parietals small, linear, remote from the orbits. Occipital of nn ordinary character, not attaining top of skull. Temiwral bone, though of great extent, not remarkably inflated. Squamosal roofing most of the cerebral cavity. Zygomata of an ordinary character, with the usual connections. Tympanic, a contracted tube. Petrosals discrete from each other, in contact with basi-occipital. Mastoid exclurled from roof of cerebral cavity. Zygomatic process of maxillary a plate with merely thickened upper border. cisofs; rostrum ntten uated, tapermg. Parietals large right-angled triangles, together as broad as the frontal, reaching orbits. Occipital reduced and of peculiar shape; a part of it mounting the top of the skull. Temporal bone unique in its enormous size and inflation, being blown up like a bladder; the swollen mastoids forming most of the occipital plane; the two temporals larger than all the rest of the skull together.* Squamosal restricted to the orbit. Zygomata thread-like in most of their extent, and greatly <lcprcsscd in position ; the malar hone abuttin u ~ against the tympanic. Tympanic, an iilflated vestibule. Pctrosals in mutual contact at their extremities, and fi surcd away from basi-occipital. Mastoid roofing most of cerebral cavity. Zygomatic process developing into a shield over much of the orbital pace. Palatal outline Rtronbcrly a ccnding P a Ia t a1 pro fil e ncar1 y straight and "If tho sonso of honriug of Dipodomys iJo co-ordinn.tcd with tl 1 n.ppa.ru.tus, it must ho oxtro.ordion.rily ncnto. 10 osseouij ( cvolopmout of tho nuuitol'y OOUES ON GEOMYS AND TBOMOMYS-OSTEOLOGY. 279 and arched anteriorly; molars far below level of zygomata. Incisors large, parallel-edged, scarcely convergmg. Anterior molar, a double prism. Root of inferior incisor protuberant posteriorly. Large erect falcate coronoid, overtop-ping condyle. &c., &c. horizontal; molars on a level with the zygomata. Incisors small, acuminate, convergent. Anterior molar, a single prism. Root of inferior incisor not prominent posteriorly. Minute prickle-like sloping coronoid, far below level of condyle. &c., &c. |