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Show 276 EXPLORATION OF THE OANONS OF THE COLORADO. GEOMYS. Superior incisors deeply channeled along the middle, with or without a fine marginal groove. Crowns of intermediate molars truly elliptical. Root of inferior incisor but little protuberant on outt:~ide of base of condylar ramuR; end of mandible thus only two-pronged, with a knob between. Zygomata widest across anteriorly, thence contracting; the width behind little, if any, greater than the intermastoid diameter of the skull. Parietals ridged along their line of union with each other. Interparietal triangular. Nasals approximately parallel-edged part way, then suddenly widening. Superficies of mastoid bone occupying nearly half the occipital surface of the skull on each side. Bullre ossero less inflated, quite acute anteriorly. Basi-occipital, in the middle, about as broad as the width of the bulla at the same point. A pair of broad cl~ep pits on the palate behinrl, extending forward to , opposite the penultimate molars. THOMOMYS. Superior incisors without median sulcus, but with a fine marginal groove (sometimes obsolete). Crowns of intermediate molars acuteedged exteriorly. Root of inferior incisors causing a protuberance on outside of base of condylar ramus nearly as high as condyle itself; end of mandible thus singularly three-pronged. Zygomata regularly convex outward, with a sweeping curve; their breadth across posteriorly decidedly greater than the intermastoid diameter of the skull. Parietals rirlged externally near the squamo-parietal suture. Interparietal rather pentagonal. Nasals widening uniformly from behind forward. Superficies of mastoid bone restricted to less than a fourth of the occipital surface on each side. Bullre ossere more inflaterl, quite ob-tuse anteriorly. . . Basi-occipital, in the middle, much narrower than the bulla at the same point. A pair of slight pits on the palatA behind, not extending beyond the ultimate molars. OOUES ON GEOMYS AND TllOMOMYS-OSTEOLOGY. 277 Pterygoids -·-~ (will be found differing appreciably from those of Tftomomys). Pterygoids appearing like a bifurcation into two thin diverging plates of a single median vertical palatal plate. In like manner, we may proceed to compare some of the principal cranial characters of Geomyidce ann Saccomyidce. Notwithstanding the unquestionably close affinity of these two families, which must stand next to each other in the system, their crania are curiously different in general appearance and details of contour. The discrepancies are, however, of a superficial character, resulting mainly from the extraordinary molding of the parts in Saccornyidre. In other words, it is a matter of mere shape, for the most part. There arc, however, some curious and more essential features, of which the enormous inflation of various elements of t.he temporal bone and peculiar zygomatic relations posteriorly are the most remarkable. Probably, going into details, a hundred actual differences between the skull:s of Gemnyidce and Saccomyidce_ might be enumerated. I shall content myself with tabulating a few of the more important of these. The compari uns are made between Geomys bu'rsarius and Dipodomys 01·di; it should be remembered, however, that the latter presents an extreme case, the average characters of the Saccomyidce being less different from those of Geomyidce. GEOMYIDlE. Skull massive, angular, in general like that of Arvicola, &c. I ntcrorbital space the narrowest part of the skull-narrower than ros-trum. Occipital region apprpaching a plane surface, without merlian emargina-tion. Na::fal bones not produced beyond SACCOMYIDJE. Skull thin and papery, the corners rounded off; the resulting general shape peculiar. Interorbital space expanded, very much broader than the rostrum. Occipital region formed chiefly of enormous bulging mastoids, with deep median cmargination. Na al bones producerl far beyond in - |