G40 MONOGRAPHS OF NOETII AMERICAN RODENTIA. 



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of characters, or even any combination of characters, will serve for the subdi- 

 vision of Spermophilus into distinct generic groups, or even satisfactorily de- 

 finable subgenera. In the large, slender-bodied, bushy-tailed forms, there is 

 an approach even to Sciurus, not only in general form but in the proportions 

 of the skull and in the small size of the first premolar; on the other hand, 

 the large, thick-bodied, short-tailed forms ditfer little either in general form 

 or in cranial details from Cynomys. Arctomys, again, is little more than an 

 exagge rat ed Spermoph ilus. 



Genetically, Arctomys may be considered as the point of departure in 

 the development of the family, as it is geologically one of the earliest forms 

 of the group. Arctomys leads readily into Spermophilus, and Spermophilus 

 into Cynomys. On the one hand, the larger, slender-bodied, bushy-tailed 

 species of Spermophilus show a tendency toward Sciurus proper, as some of 

 the other large forms lead toward Arctomys and Cynomys; some of the 

 smaller species, with large ears, long, flat, bush}' tails, small first premolar, 

 and general Sci urine form, grade insensibly into Tamias, while Xcrus may 

 have come off from the short-eared, terete-tailed phase of the same rather 

 heterogeneous group. The hiatus between some of the forms of Tamias 

 and the more arboreal Sciuri is by no means striking, while Pteromys and 

 Sciuropterus seem to be only more specialized types of the strictly 

 Sciurine form, in which the already highly specialized arboreal adaptation is 

 carried still further through the addition of a supporting membrane, enabling 

 them to assume, an imperfect mode of aerial locomotion. It hence follows 

 that in Arctomys we find the most generalized type of the family; in Pte- 

 romys and Sciuropterus, the most specialized. Through Arctomys, also, we get 

 a distant affiliation with other types of the Sciuromorphs, especially with 

 Castoridte and Haplodontidce, while Anomalurus may be a still further special- 

 ized offshoot in the direction of Pteromys. 



SYNOPSIS OP THE GENERA. 



I. Skull and dentition essentially as in Sciurus ; upper premolars two, the first minute ; limbs uniti d 

 by a furred membrane, supported auteriorly by a slender bone articulating with the carpus ; 

 ears large, sparsely furred ; no cbeek-pouches ; nail of pollex rudimentary ; pelage very soft and 



furry ; tail very broad ; size small Soiwopterui. 



II. Similar in external character to the preceding, but with important differences in dentition, in 

 respect to the relative size of the grinding-teeth and their structure; the posterior premolar 

 and the last molar being much smaller than the first and second true molars, instead of nearly 

 equalling them in size, and the triturating surface of the molars having a more complex system of 

 tubercles and ridges; size large rterum^s. 



