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FAMILY 59.-SACCOMYIDÆ. (6 Genera, 33 Species.)
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. i. 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.
FAMILY 60.—CASTORIDÆ. (1 Genus, 2 Species.)
1 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.
Esctinct Castorida.-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.
FAMILY 61.—SCIURIDÆ.—(8 Genera, 180-200 Species.)
1 2.3.4 1.2.3 1 The Squirrel family, comprehending also the marmots and prairie-dogs, are very widely spread over the earth. They are especially abundant in the Nearctic, Palæarctic, and Oriental regions, and rather less frequent in the Ethiopian and Neotro. pical, in which last region they do not extend south of Paraguay. They are absent from the West Indian islands, Madagascar, and Australia, only occurring in Celebes which doubtfully belongs to the Australian region. The genera are as follows :
Sciurus (100—120 sp., including the sub-genera Spermosciurus, Xerus, Macroxus, Rheithrosciurus, and Rhinosciurus), comprises the true squirrels, and occupies the area of the whole family wherever woods and forests occur. The approximate number of species in each region is as follows: Nearctic 18, Palæarctic 6, Ethiopian 18, Oriental 50, Australian (Celebes) 5, Neotropical 30. Sciuropterus (16—19 sp.), comprises the flat-tailed flying squirrels, which range from Lapland and Finland to North China and Japan, and southward through India and Ceylon, to Malacca and Java, with a species in Formosa ; while in North America they occur from Labrador to British Columbia, and south to Minnesota and Southern California. Pteromys (12 sp.), comprising the roundtailed flying squirrels, is a more southern form, being confined to the wooded regions of India from the Western Himalayas to Java and Borneo, with species in Formosa and Japan. Tamias (5 sp.), the ground squirrels, are chiefly North American, ranging from Mexico to Puget's Sound on the west coast, and from Virginia to Montreal on the Atlantic coast; while one species is found over all northern Asia. Spermophilus (26 sp.), the pouched marmots, are confined to the Nearctic and Palæarctic regions ; in the former extending from the Arctic Ocean to Mexico and the west coast, but not passing east of Lake Michigan and the lower Mississippi; in the latter from Silesia through South Russia to the Amoor and Kamschatka, most abundant in the desert plains of Tartary and Mongolia. Arctomys (8 sp.), the marmots, are found in the northern parts of North America as far down as Virginia and Nebraska to the Rocky Mountains and British Columbia, but not in California; and from the Swiss Alps eastward to Lake Baikal and Kamschatka, and south as far as the Himalayas, above 8,000 feet elevation. Cynomys (2 sp.), the prairie-dogs, inhabit the plains east of the Rocky Mountains from the Upper Missouri to the Red River and Rio Grande (Plate XIX., vol. ii. p. 129). Anomalurus (5 sp.), consists of animals which resemble flying-squirrels, but differ from all other members of the family in some points of internal structure. They form a very aberrant portion of the Sciuridæ, and, according to some naturalists, a distinct family. They inhabit West Africa and the island of Fernando Po.
Extinct Sciurida.--These are tolerably abundant. The genus Sciurus appears to be a remarkably ancient form, extinct species being found in the Miocene, and even in the Upper Eocene formations of Europe. Spermophilus goes back to the Upper Miocene; Arctomys to the Newer Pliocene. Extinct genera are, Brachymys, Lithomys and Plesiarctomys, from the European Miocene, the latter said to be intermediate between marmots and squirrels.
In North America, Sciurus, Tamias, and Arctomys occur in the Post-pliocene deposits only. The extinct genera are Ischyromys, from the Upper Miocene of Nebraska ; Paramys, allied to the marmots, and Sciuravus, near the squirrels, from the Eocene of Wyoming
Here we have unmistakable evidence that the true squirrels (Sciurus) are an Old World type, which has only recently entered North America; and this is in accordance with the comparative scarcity of this group in South America, a country so well adapted to them, and their great abundance in the Oriental region, which, with the Palæarctic, was probably the country of their origin and early development. The family, however, has been traced equally far back in Europe and North America, so that we have as yet no means of determining where it originated.
FAMILY 62.—HAPLOODONTIDÆ.—(1 Genus, 2 Species.)
The genus Haploodon or Aplodontia, consists of two curious rat-like animals, inhabiting the west coast of America, from the southern part of British Columbia to the mountains of California. They seem to have affinities both with the beavers and marmots, and Professor Lilljeborg constitutes a separate family to receive them.
FAMILY 63.-CHINCHILLIDÆ. (3 Genera, 6 Species.)
The Chinchillidæ, including the chinchillas and viscachas, are confined to the alpine zones of the Andes, from the boundary of Ecuador and Peru to the southern parts of Chili; and over the Pampas, to the Rio Negro on the south, and the River Uruguay on the east. Chinchilla (2 sp.), the true chinchillas, are found in the Andes of Chili and Peru, south of 9° S. lat., and from 8,000 to 12,000 feet elevation (Plate XVI. vol. ii. p. 40); Lagidium (3 sp.), the alpine viscachas, inhabit the loftiest plateaus and mountains from 11,000 to 16,000 feet, and extend furthest north of any of the family; while Lagostomus (1 sp.), the viscacha of the Pampas, has the range above indicated. The family is thus confined within the limits of a single sub-region.
Extinct Chinchillidæ.-Lagostomus has been found fossil in the caves of Brazil, and in the Pliocene deposits of La Plata. The only known extinct forms of this family are Amblyrhiza and Locomylus, found in cavern-deposits in the island of Anguilla, of Post-Pliocene age. These are very interesting, as showing the greater range of this family so recently; though its absence from North America and Europe indicates that it is a peculiar development of the Neotropical region.
FAMILY 64.–OCTODONTIDÆ. (8 Genera, 19 Species.)