RODENTI A CASTORINAE APLODONTIA. 35 1 



I ain unable to speak further than that both are destitute of the post-orbital process. The 

 distinct rounded ante-orbital foramen, in the base of the zygoma, resembles somewhat that of 

 Tamias. 



Skull. The skull of Aplodontia, with the exception of the rootless molars and the lack of a 

 post-orbital process, offers many points of close relationship to that of Sciuridae generally. 

 It is, however, most remarkably depressed or flattened, the planes of the upper and under 

 surfaces almost parallel. Viewed from above, its shape, exclusive of the muzzle, is that of an 

 equilateral triangle ; the occiput, almost the widest part of the skull, nearly equals the greatest 

 distance between the zygomata ; its plane is vertical and perpendicular to that of the palate. 

 The occipital crest is well developed, with a decided fossa anterior to it on each side of the top 

 of the skull ; its two halves curve slightly forward from the exterior to their place of meeting 

 on the sagittal suture. The upper part of the cranium is much rounded or arched. There is, 

 however, a decided spherically concave depression between the orbits, just anterior to the region 

 of greatest contraction. This contraction is very great, situated posterior to the centre of the 

 skull, which at this point is narrower than the nasal bones, and only about half the distance 

 between the outer borders of the alveoli of the second anterior molars, or equal to the length of 

 the crowns of the second and third molars. The bony orbits are very large ; viewed from 

 above, they are somewhat right-angled triangled, the shortest side the antero-interior, about 

 two-thirds the length of the postero-interior, which is nearly straight. The hypothenuse is 

 considerably curved, and formed by the inner border of the malar bone. The muzzle is large 

 and much rounded ; indeed, its vertical section across the middle would be nearly circular. 



Owing to the extreme age of the specimens it is impossible to trace out the course of the 

 sutures, and consequently to indicate the connexions of the malar. The lachrymal bone may 

 be identified, however, in a tubercle at the anterior angular extremity of the orbit, and imme 

 diately within and behind this is a distinct notch, with a small process, which represents the 

 ante-orbital ; the edge of the frontal then curves rapidly inwards, as described, without any 

 post-orbital process whatever. The cavity of the cranium is consequently very small, compared 

 with the true squirrel. 



The posterior root of the malar bone is thin, and passes out from the temporal in a horizontal 

 plane, where it is twisted more vertically and passes forward ; its lower edge, horizontal or 

 slightly concave, extends beyond the anterior molar, and then descending very little reaches 

 the plane of the palate. The palate itself is nearly horizontal from the incisors ; although an 

 terior to the molars it slopes off on each side of the middle region. The ante-orbital foramen is 

 not large ; it is oval, the longer axis directed obliquely upwards, outwards and forwards ; the 

 two branches of the zygoma forming its outer walls are nearly equal. The incisive foramina 

 are large, situated just back of the incisors, and are one-third as long as the distance from in 

 cisors to molars. The largest palato-maxillary foramina are between the centres of the fourth 

 molars ; the palate between the molars is perfectly plane and extends behind the last molar, by 

 its own length terminating in a concave edge behind. The glenoid cavity is remarkably long, 

 broad and shallow. 



The auditory bullae are quite small, but open rapidly into very wide auditive tubes, much 

 larger, with thinner walls, than in the beaver ; these extend horizontally beyond the posterior 

 angle of the zygoma. 



The inner lines of molar alveoli are about parallel ; the outer are slightly divergent ante 

 riorly, owing to the increase in size of the molars from behind from the fifth to the second in 



