342 THE HOLARCTIC REGION. [CHAP. 



which, as in the eastern division, are very numerous, comprise one 

 family, as well as several genera, restricted to this area. In the 

 Sciuridce the marmot- like genus Cynomys ranges into the Hoi- 

 arctic, but is considered by Dr Merriam as chiefly characteristic of 

 the Sonoran region; and the same is the case with the white- 

 footed mice (Sitomys) of which there is but a single Holarctic 

 representative, while the Sonoran species are very numerous and 

 also with the wood-rats (Neotoma), of which a sub-genus is 

 restricted to the Holarctic. The family peculiar to the region is 

 that of the Haplodontidtz, or sewellels, represented by two species 

 of the genus Haplodon, from the districts west of the Rocky 

 Mountains. Closely allied to the squirrels, these rodents are 

 distinguished from the latter by the absence of postorbital pro- 

 cesses to the frontal bones of the skull, the depressed skull, and 

 the rootless, or hypsodont, cheek-teeth ; all these characters indi- 

 cating a more specialised type. In the Muridce, the voles of the 

 genus Phenacomys connect the more typical members of the 

 group with cricetines like the wood-rats (Neotoma). Several 

 species have been described. A more southern type is the single 

 representative of the allied genus Synaptomys, distinguished by its 

 grooved upper incisors \ its molar teeth resembling those of the 

 lemmings, while its skull is of the same structure as in the true 

 voles. According to Dr Merriam, this animal is restricted to the 

 southern part of the Holarctic area, or what he terms the Transi- 

 tion region. In the same great family the well-known aquatic 

 musk-rat, or musquash (Fiber), may be considered an Holarctic 

 type, since it is found in the " barren-grounds " on the borders of 

 the Arctic sea, although it ranges southwards into the Sonoran. 

 Closely allied to the voles, with which it agrees in the characters 

 of the skull and teeth, this animal differs by the long, compressed, 

 nearly naked, and reticulate tail ; the naked-soled feet being partly 

 webbed, and the whole body adapted to an aquatic mode of life. 

 Its fossil remains occur in the Plistocene of the United States. It 

 is noteworthy, as a negative characteristic of the Holarctic area, 

 that no members of the exclusively New World family Geomyidce 

 are found within its limits. On the other hand, in the family 

 Dipodida, the j urn ping-mice of the genus Zapus, of which several 

 species are recognised by North American zoologists, are solely 



