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U. S. P, R. R. EXP. AND SURVEYS—ZOOLOGY-GENERAL REPORT. 
The most striking peculiarities, however, of this genus are discernible in the molars. The 
outlines of the molar series, instead of being nearly straight and parallel, as in Arctomys , or 
curved with parallel chords, as in Spermophilus and Sciurus, are curved and very strongly 
divergent anteriorly and approximated behind. Thus the narrowest part of the palate is be¬ 
tween the fifth molars, and measures only half as much as the widest portion between the first. 
There is only a slight approximation to this anterior divergence in Spermophilus. The molars, 
too, are very large, and occupy a long interval of the skull, equal to more than one-fourth of 
the total length, while in Arctomys and Spermophilus this space is less than one-fifth. The in¬ 
terval between the anterior molar and the incisors is less than the length of the line of molars, 
while in Arctomys and Spermophilus it is considerably greater. 
The first upper molar is very nearly as large as the second, particularly in the section made 
by a horizontal plane. It, however, has but one root, and is inserted obliquely forwards and 
inwards ; its crown is peculiarly marked by a compressed crest rising at an angle with a lower 
ridge on either side, separated by a narrow valley ; these are parallel with the central one. 
The general direction of these ridges is forwards and inwards, and their lines, if produced, 
would meet in the middle of the incisive foramina, instead of being nearly transverse, as in some 
Spermophilus and Arctomys. The second, third, and fourth molars increase but little in size 
posteriorly, and are more narrowed internally than in the allied genera. The fifth, however, 
is much larger and in the shape of a quarter circle, with the two perpendicular radii forming 
the anterior and interior margins of the tooth. Its surface exhibits three distinct valleys, in¬ 
stead of the two usually seen in the slightly worn tooth of the Sciurinae. 
The lower molars correspond in development to the upper. They are shorter in proportion to 
their width than in other Sciurinae, especially the first, while the reverse prevails in the last 
molar, which is considerably longer than wide. Viewed laterally, the anterior lobe of each 
tooth is much larger, higher, and more pointed than the posterior, while in other Sciurinae 
these are of nearly equal height. The shape of the jaw, too, is different. 
The upper incisors are large and considerably rounded on the external angle. They are twice 
as deep as broad, with faint shallow striae on the anterior face. The enamel shows laterally on 
about one-third of the side of this incisor. In Arctomys , the incisor is nearly as broad as deep, 
and the enamel covers more than half the side. 
From the above description it will be seen that the skull of this genus differs very greatly 
from its allies, more, indeed, than any of these do among themselves. In the totality of 
characters the approximation to Arctomys is considerably the closest. From this and Spermo¬ 
philus it differs in the divergence anteriorly of the line of molars ; the great size of the molars, 
especially the first; the greater size and complexity of the posterior upper one; the strong 
convergence anteriorly of the zygomata, &c. 
The ears of this genus are very short, in fact, nearly rudimentary. The eyes are moderately 
large. The tail is short, one-fourth to one-eighth the length of body, thinly haired, and the 
lateral hairs longest. The body is very thick and squat. The feet are large ; the claws well 
developed ; those on the forefeet much larger than the hinder ones, and distinctly five in 
number ; that on the thumb unusually large. The pupil of the eye, in an alcoholic specimen, 
appears slightly vertical, but it seems circular in a living animal. The ridges of the palate are 
considerably more numerous between the molars than the molars themselves. The pouches are 
very shallow, and do not extend much deeper than in Arctomys, not as far back as the eye, or 
scarcely half an inch. 
