640 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
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 Sciwrus, 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 differ little either in general form 
or in cranial details from Cynomys. Arctomrys, again, is little more than an 
exaggerated Spermophilus. 
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 Sccwrus proper, as some of 
the other large forms lead toward Arctomys and Cynomys; some of the 
smaller species, with large ears, long, flat, bushy tails, small first premolar, 
and general Sciurine form, grade insensibly into Tamas, while Xerus 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 Tamas 
and the more arboreal Sciwri is by no means striking, while Péeromys and 
Sciuroplerus 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 aérial locomotion. It hence follows 
that in Arctomys we find the most generalized type of the family ; in Pée- 
romys and Sciuropterus, the most specialized. Through Arctomys, also, we get 
a distant affiliation with other types of the Sciuromorphs, especially with 
Castoride and Haplodontide, while Anomalurus may be a still Be special- 
ized offshoot in the direction of Pteromys. 
SYNOPSIS OF THE GENERA. 
I. Skull and dentition essentially as in Sciwrus ; upper premolars two, the first minute ; limbs united 
by a furred membrane, supported anteriorly by a slender bone articulating with the carpus; 
ears large, sparsely furred ; no cheek-pouches; nail of pollex rudimentary ; pelage very soft and 
furry’; tail very broad *ysize small: s.e..ce oe te ee ee eee Sciuropterus, 
Il. 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 pren.olar 
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 
SSeS aS Conta oe Honoit cio oon naneecorense! Pa vin Go: 
