85 



by the mastoid. In Geomys, at anj- rate, the mastoids take as much part 

 iu the occipital surface as the occipital bone itself. The upper border 

 of this surface is a uearly regular arch from oue squamosal angle to the 

 other. The lower outline is likewise a curve, with its convexity down- 

 ward, but its regularity is brokeu by the nick of the foramen magnum in 

 the middle, the protuberance of the condyles next, similar paroccipital 

 processes next, and after a little interval the mastoid processes. ]5arring 

 these irregularities of detail, the general occipital surface is elliptical in 

 shape. In the middle, and niking the lower limb of the ellipse, is the 

 foramen magnum, nearly all of which is vertical, and consequently not 

 foreshortened iu this view. 



Viewed from below, the general contour is substantially like that 

 presented from the opposite inspection, and we need only attend to 

 details. The first feature is the incisive foramina — very small slits lying 

 wholly in the intermaxillary bones, yet nearer to the molars than to the 

 incisors, so great is the production of the rostrum. The palate proper,* 

 i. e., the intermolar portion, is extremely contracted, its width anteriorly 

 being no greater than that of one of the molars. It widens a little back- 

 ward. It is deeply twice furrowed, having a strong median ridge sepa- 

 rating the furrows, and strong alveolar ridges on either hand. Pos- 

 teriorly, there is a pair of deep pits extending to opposite the penultimate 

 molars, and divided by a strong ridge. The palatal plate upon which 

 these pits are constructed reaches considerably back of the molars in 

 Geomys; less so in Thomomys. The general resemblance of the parts 

 ito SDme Arvicolince is strong. The pterygoids are thin, vertical, and 

 somewhat circular jilates, divaricating a little posteriorly, and abutting 

 against the tips of the bulla ossea. They appear like a bifurcation of 

 the median palatal ridge just mentioned. The post-palatal parts being 

 contracted, like the palate itself, and compressed into small space, it is 

 inot easy to fully appreciate the conformation of the parts, and still less 

 so to describe it. Moreover, the lamellar pterygoids are often broken 

 off in careless preparation of the skull, and in such way that scarcely a 

 suggestion of their former presence is left. Behind the pterygoids, the 

 conspicuous bullje osse?e appear convergent anteriorly to touch the 

 former, prolonged into a tube exteriorly. Between them, the basi- 

 occipital space is cuneiform, (especially in Thomomys — more nearly quad- 

 rangular in Geomys,) with a median ridge and lateral depressions, nicked 

 behind by a small portion of the foramen magnum. The skull finishes 

 behind by an irregular curve, substantially the same as that described 

 in speaking of the occipital plane. 



In all but the oldest animals, the following sutures, or at any rate 

 traces of them, persist : internasal, uaso-intermaxillary, maxillo-in- 

 termaxillary, fronto-nasal, fronto-iutermaxillary, and fronto-maxillary ; 

 maxillo- molar, squamo-parietal, squamo-malar, squamo-mastoid, occi- 

 pito-mastoid, occipito-petrosal ; basi-occipito-sphenoid; and there is 

 fissured separation of the petrosal and tympanic from the squamosal. 

 The various intricate relations of the palatals, and of the " sphenoid" 

 as a whole, are iuapi)reciable iu the adult skull. Detailed relations of 

 such of the individual boues as can be made out from the material be- 

 fore me, here follow : 



The nasals reach back to a point opposite the anterior root of the 

 zygoma, but extend little, if any, iu the other direction, beyond the 

 intermaxillaries. For two-thirds their extent they are narrow and ap- 

 proximately parallel in the examples of Geomys before me, and then 

 rapidly expand. In all the Thomomys I have seen, they widen regu- 



* The lon^ iipward-slopiDg anterior part of palate is not " palate" at all. In life, it is 

 ^altogether outside the mouth, like the superior incisors, and covered with furry skin. 



