The Saccomyidæ, or pouched rats, are almost wholly confined to our second Nearctic sub-region, comprising the Rocky Mountains and the elevated plains of Central North America. A few species range from this district as far as Hudson's Bay on the north, to South Carolina on the east, and to California on the west, while one genus, doubtfully placed here, goes south as far as Honduras and Trinidad. The group must therefore be considered to be pre-eminently characteristic of the Nearctic region. The genera are,--_Dipodomys_ (5 sp.), North Mexico, California, the east slope of the Rocky Mountains to the Columbia River, and one species in South Carolina; _Perognathus_ (6 sp.), North Mexico, California, east slope of the Rocky Mountains to British Columbia; _Thomomys_ (2 sp.), Upper Missouri, and Upper Columbia Rivers to Hudson's Bay; _Geomys_ (5 sp.), North Mexico, and east slope of Rocky Mountains to Nebraska (Plate XIX., vol. ii. p. 129); _Saccomys_ (1 sp.), North America, locality unknown; _Heteromys_ (6 sp.), Mexico, Honduras, and Trinidad. _Geomys_ and _Thomomys_ constitute a separate family Geomyidæ, of Professor Carus; but I follow Professor Lilljeborg, who has made a special study of the Order, in keeping them with this family. In the Post-Pliocene deposits of Illinois and Nebraska, remains of an existing species of _Geomys_ have been found. {234}FAMILY 60.--CASTORIDÆ. (1 Genus, 2 Species.) GENERAL DISTRIBUTION. __________________________________/\__________________________________ / \ NEOTROPICAL| NEARCTIC |PALÆARCTIC | ETHIOPIAN | ORIENTAL | AUSTRALIAN SUB-REGIONS|SUB-REGIONS|SUB-REGIONS|SUB-REGIONS|SUB-REGIONS|SUB-REGIONS. -----------+-----------+-----------+-----------+-----------+------------ | | | | | -- -- -- --|1. 2. 3. 4 | 1 -- 3 -- |-- -- -- --|-- -- -- --|-- -- -- -- | | | | | The Beavers, forming the genus _Castor_, consist of two species, the American (_Castor canadensis_) ranging over the whole of North America from Labrador to North Mexico; while the European (_Castor fiber_) appears to be confined to the temperate regions of Europe and Asia, from France to the River Amoor, over which extensive region it doubtless roamed in prehistoric times, although now becoming rare in many districts. _Extinct Castoridæ._--Extinct species of _Castor_ range back from the Post-pliocene to the Upper Miocene in Europe, and to the Newer Pliocene in North America. Extinct genera in Europe are, _Trogontherium_, Post-Pliocene and Pliocene; _Chalicomys_, Older Pliocene; and _Steneofiber_, Upper Miocene. In North America _Castoroides_ is Post-Pliocene, and _Palæocastor_, Upper Miocene. The family thus first appears on the same geological horizon in both Europe and North America.