330 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 heing nearly straight and parallel, as in Arctomys, or 

 curved with parallel chords, as in Spermopldlus 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 

 mwards ; 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 stria? 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. 



