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Show I I I I"' 268 EXPLORATION OF THE OARONS OF THE OOLORADO. though specimens differ in this respect, owing to a variable curve of these parts. In adult Geomys, the case is as stated, the zygomata converging a Jittle backward in a nearly struight line, so that posteriorly their width apart is little if any greater than the intermastoid diameter of the skull. In Thonwm. 7Js, there is a more decided outward convexity of these arches, and their greatest width apart is nearly at their middle-if anything, posterior to this, and at any rate the width here decidedly surpu scs the intermastoid diameter. In front, the zygomatic plates of t.he maxillaries start out at nearly a right angle with the long axis of the skull; behind, the zygomata curve rather abruptly into the squamosal. There is a deep abrupt emargination behind the posterior root of the zygomata, between this and. the postero-lateral corner of the skull; in the recel:'s, the tubular bo?y meatus auditorius appears protruding in this view. The lambdoidal crest, forming the posterior boundary of the skull, is a ' light curve, more or less irregular; mo t of it is squamosal, fur the occipital bone rises to this crest for only a short d istancc. 'fhe narrowest part of the skull is bet ween the orbitt:S, where the width i less tha~ the diameter of the rostrum. The irregularly pyriform figures, circumscn. bed by the zygomata and walls of the cranium, arc of large size; no orbtts ~roper arc defi~ed in the general orbital space, owing to dcficieucy of ~oth ~~ c- and ~ost-.orb1tal processes. The dome of the ccrebrul cavity is but htt.le wflated; 1.ts s~des seem somewhat pinched, there being a decided though sh.a llow co.n cavity JUSt above the zygom'a tic spur of the squamo a1 ; an d a shght bulgmg anteriorly on each side at the usual site of pot:St-orbital pro - cesses. The median line of the cerebral roof. in an old Geom . 1 11 · · c1 h' · . ' ys s {\I , tR n rt ge; t 1S ndg.e btfurcates anteriorly to send a curved le g 1c0 1. war d nne i out-w. ard to . the orbttal margins; and behind enlarges a l'tttle to recm.v e a smu 11 mterpanetal. In various Thomomys skulls of d' ffi t 1 . 1 eren ages, the squarnosnls eave a rectangular mterval occupied by sm•tll na.. 1 ]' . • • L • 11 ow Y mear panetals · and mstead of a smgle median ridge, there are two parallel ridacs with ad ' d' interval Tl · d f th o ' cpreRse . le Sl es o e rostrum are straight and parallel, the cd re bcinc.r the swollen track of the superior incisor . The end ·, .• . II g 0 th · f I:s ver ~lea y truncate· e tips o the nasals and intermaxillaries and the face" of tl . . I •. ' all b t ' :s 1C 1 OCISOfS lCtng a ou m oue perpendicular plane. The widtl f . more than half il.s length. l o the rostrum ls rathel OOUES ON GEOMYS AND TBOMOMYS-OSTEOLOGY. ~69 Viewed in profile, the sku ll shows an ulmo.·t perfectly straight dorsal outline from the occipital protuberance io a point just in advance of the orbits. Here is the highest point of the skull, whence the profile of the rostrum slopes gently downward, ending abruptly by vertical truncation. Likewise, the posterior or occipital outline is straight, or nearly so, and at a right angle with the uperior surface. Likewise, again, the inferior surface of the skull, in all that part lying behind the pterygoicls, presents a nearly straight and horizontal profile, at right angles with the occipital plane. Neither bulla ossea nor paroccipital nor condyle is sufficiently developed to interfere with the straightnrss of outline and rectangularity which nll the back part of the t;kull presents to the ide view. The rest of the under outline of the skull oonsists of the palatal profile as n whole. 1:'his consi ts anteriorly of a deep (semi-oval) concavity; there is an abrupt rise from the incisive alveolus, and then a long gradual curve ~doping far backward and downward to the molar alveolus; while the 'irong obliquity of set of the anterior molars protracts this same curve to the tips of the teeth. The molar alveolp.r border is very short., and rather oblique, being lowest behind. 1'he enormous arched interval between the inci 'Ors unci molars is highly characteristic, as i also the low po ition of the molars-the teeth dip below a line drawn from the tips of the incisors to tho foramen magnum. Behind the palate, flange-like pterygoicls slope up to the basi-occipital plane. In this view, the zygomata are seen to dip but slightly downward. Their point of greatest deflection lie high above a line drawn from the inci ivc alveolu · to the occipital condyle-in fact, even above a line from the end of the nasal bones to the same point; at their lowc~ t point, they are still on a level with the ment.us, and they ~carcely dip more than half-way from the top of the ~Skull to the level of the molar crowns. For the rest, notable points of the profil~ view of the skull are the small size and peculiar position of' the "antcorbital '' fi>ramen, here situate low down and far forward in the maxillary, near its antero-inferior angle; a deep pit, but not perforation, behind the zygomatic plate of the maxillary; extensive lacerate foramina of exit of nerves entering the orbit from the brain; similar fissured vacuities between the bulla ossea and the quamo al. The foreshorten d tubular me~ttus is ~ccn in the deep reces between the posterior root of the zygoma and the postero-inferior angle of the squamosal. |
