338 U. 8S. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
ARCTOMYS, Schreber.! 
Arctomys, ‘*ScureBer, Saugthiere, IV, 1792.’’ 
Avup. & Bacu. N. Am. Quad. I, 1849, 16. 
Sp. Cu.—Size large; body very thick and depressed ; thumb very rudimentary, armed with a small flat nail, not a claw ; 
soles entirely naked ; cheek pouches very shallow ; pupil rounded? 
It is in this genus that we find the largest members of the Sciurinae, some of them, indeed, 
taking the second or third rank in size among North American rodents, being exceeded only by 
Castor and Hystrix, and one species of hare, the body is thick and massive, the ears distinct 
and well shown above the fur, though short and rounded, The tail is bushy and full, but nearly 
cylindrical, the hairs being as long above and below as laterally. The thumb is rudimentary, 
with a short flat nail, nota claw. The outer finger of the hand extends as far as two-thirds 
the length of the palm, and in all the feet the claw of the outer toe reaches beyond the base of 
the one next to it. 
The skull is much more flattened than in the spermophiles, in fact its upper outline is not 
only nearly straight, but it is decidedly concave between the orbits. The post-orbital processes 
are very large, the distance between their apices equal to the greatest width of the back part of 
the head ; their posterior margin is rectilinear and perpendicular to the axial line of the skull, 
until it passes gently into the temporal ridge of the parietal bone, which is quite prominent 
and.concave externally. There is a distinct semicircular notch behind the subtubercular ante- 
orbital process of the frontal; the nasal bones extend back nearly as far as this notch. The 
malar bone is twisted angularly, and shows externally three principal faces ; anteriorly, it is 
wedged between the maxillary and the laehrymal bones. The palate is broad and rather plane, 
though flattened concave between the molars—the bottom of the palate on a level with the in- 
ferior edge of the root of the zygoma. The external outlines of the upper molars are rectilinear 
and parallel in the adult—curved in the young ; the inner outlines are slightly divergent ante- 
riorly in the young. The plane of the grinding surface is nearly horizontal at first, but twisted 
outwards more and more obliquely behind. There is no very essential difference in the shape 
of the lower jaw of Sciwrus and :Arctomys. 
The incisors are much less compressed than in Sciurus, considerably rounded anteriorly and 
laterally ; the enamel extends half way round the side. There are faint indications of numer- 
ous fine striz on the enamel. The crown of the anterior upper molar is about half the area of 
the second, and its summit is on the same level with the rest. The other teeth are nearly equal, 
but increase gradually posteriorly. The first molar has one root, the others three, each; the 
crown of the first molar, in its most perfect state, exhibits a semicircular marginal ridge on the 
inner, anterior, and posterior sides, thus encircling nearly three-fourths of the crown; within 
this curve rises a transversely elongated tubercle. 
In the young skull the deciduous first upper molar is much smaller than its successor; the 
crown however, is not a simple conical tubercle, but shows a distinct longitudinal ridge, with a 
pit on either side; in this case, however, the direction of the ridge is nearly with the line of 
' This is the authority given by Agassiz in Nomenclator Zoologicus. I find the name used, however, by Gmelin, in 1788. 
It is probable that Schreber made it in one of the earlier numbers of his volume IV, which was not completed nor furnished 
with a title until 1792. 
