C40 



MONOGRAPHS OF NORTH AMERICAN RODENTIA, 



h 'M 



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

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

 finable subgenera. In the large, slender-bodied, bushy-tnilcd 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 differ little either in general form 

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

 exaggerated SpermopMlus, 



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 tiie group. Arctomys leads readily into SpermophUus, and SpermopMlus 

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

 species of SpermophUus show a tendency toward Sciurus proper, os 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 Tamias, while Xerus may 

 liave come off from Ihe 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 

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

 tliat 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 

 Casloridte and Haplodontuke, while Anomalurus may be a still further special- 

 ized offshoot in the direction of Pteromys, 



BYN0FHI8 OF TUK QENEBA. 



I. Sknil and deDtition eaiientlMUy as in 5ciiirii< ; nppxr premolara two, the Ant minate ; llmba notlcd 

 by a furred membrane, gnpported anteriorly by a alender bone artionlating with the carpna ; 

 eara large, gpaiaely forred ; no oheek-ponchea ; nail of poUex nidinientary ; pelage rery toft and 



furry; tail very broad ; size imall Sotunplent. 



II. Similar in external obaraoter to the preceding, but with important differeucea in dentition, in 

 Tespeottothe relative sixe of the griuding-teeth and their atrootnre; the poeterior pivn.olar 

 and the lost molar lieing much amaller than the first and aeooud true molan, iuetead of nearly 

 equulliDgthem in size, and the triturating anrfaceof the molara having a moieoomplex ayatem of 

 tubercle* and ridgea; sixe large Ptennni. 



