352 
. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY-GENERAL REPORT. 
the upper jaw, from fourth to first ia the lower. The first molar above is very small and 
situated against the antero-internal corner of the second ; it is oval and simple. All the molars 
are prismatic and rootless. The general shape of their section in the upper jaw is circular, the 
inner side quite so ; the outer, however, with a prominent salient angle, on either side of which 
is a slight concavity. The second upper molar is larger, and has a concavity on the antero- 
internal corner. The lower molars are rather longer and narrower, somewhat like the upper, 
except that the salient angle is on the inner side ; and there is a decided groove on the outside, 
anterior to the middle of the tooth. The molars of both jaws are situated much further hack 
than usual, the middle of the skull being opposite the junction of the second and third ; the 
line of the molars is a little more than one-fourth the length of the skull. The incisors of both 
jaws are much rounded anteriorly : the section of the upper nearly circular, of the lower sub- 
triangular. 
The lower jaw is very remarkable in its shape. The inner edges of the molars of opposite 
sides are perfectly parallel. The descending ramus is twisted so as to be perfectly horizontal 
behind, its postero-inferior edge being a straight line, almost exactly perpendicular to the ver¬ 
tical plane of the axis of the skull. The inferior angle of the descending ramus, now become 
interior, is extended inwards until those of opposite sides are separated by the interval between 
the molars. The condyles have their long axis transverse, and are convex externally ; the condy¬ 
loid process is low, its upper edge straight and nearly horizontal, the coronoid process rising 
anterior to it, very broad, high and falciform. The'junction of the first and second molars is 
about midway between the end of the incisors and the back part of the condyles. 
From the above description of the skull of Aplodontia, it will be seen that there is a remark¬ 
able predominance of horizontal planes and straight lines perpendicular to the three co-ordinate 
planes. Thus the plane of the occiput is vertical and perpendicular to that of the base of the 
skull, which itself, as nearly as may be, is horizontal. The general outline of the top of the 
head is parallel with that of the bottom ; the lower edge of the occiput is horizontal and perpen¬ 
dicular to the horizontal axis of the skull. The lower edge of the zygoma is nearly rectilinear 
and parallel with the plane of the palate. In the lower jaw a vertical plane would be tangent 
to the condyles and the nearly horizontal posterior edge of the inferior ramus. The planes of 
the broad coronoid processes are vertical, though inclined to each other. 
From the preceding description, therefore, of Aplodontia, it will, I think, be clearly evident 
that its affinities are decidedly with the Sciuridae. 
The genus Aplodontia was first established by Richardson as properly expressing the charac¬ 
ters of the animal. Somewhat faulty in its etymological construction, the name has been 
variously modified by purists ; but as one well established by use, I have considered it best to 
adhere to the one originally given by Richardson. 
The genus Anisonyx was established by Rafinesque to embrace the general characters of the 
“ burrowing squirrel” of Lewis and Clark, as given by them, but entirely misapprehended by 
him. As the diagnosis does not really apply in many important points, it will be necessary to 
reject the name, especially as it comes mixch nearer to Cynomys, a genus of the same author, 
and well characterized by him about the same time. 
