352 . S. P. R. R. EXP. AND SURVEYS ZOOLOGY GENERAL REPORT. 



the upper jaw, from fourth to first in 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 back 

 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 ; thecondy- 

 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 coudyles. 



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 Kichardson 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 much nearer to Cynomys, a genus of the same author, 

 and well characterized by him about the same time. 



