Extent and Classification.— This is by far the largest order of irammab, comprising more than 2,000 species: Rodents are found in all parts of the world, but are poorly represented in the Australian 'region and on Madagascar and are altogether absent from certain oceanic islands. In South America they, reach their maximum development. With a very •few ex ceptions, such as the house-rats and the musk rat, they are exclusively herbivorous, and dif ferent. species subsist upon grains, nuts, barks, roots, herbage, etc. Notwithstanding their great structural similarity the rodents have fitted themselves to the most varied conditions and present many remarkable adapt• eations and interesting habits. Most of them are terrestrial; many, like the mole-rats, rabbits and prairie marmots, are burrowers: some, all the beaver and muskrat, are aquatic; many squirrels and others are arboreal. nm, leap, climb or, like the flying squirrel and Ana• malunts, sail through the air on. skinny para., chutes. Some, as the woodchuck, are many are gregarious, like the rats and mice, colonial like the rabbit and prairie marmot, or even, like the beaver, form co-operative so-, cieties and exhibit wonderful building instincts.
Migratory movements of large parties of rodents are not infrequent, the most remarkable of which are the well-known migrations of the lemmings. Although generally of small size, the, rodents have gained a dominance both of species and individuals through their great fecundity and their ability to gnaw out a living beyond the reach of most animals. Their wari4 mess and secretiveness have aided no little toward their preservation. though they form the chief subsistence of a host of snakes, birds of prey and small carnivorous mammals. }Lillian Interests are affected by these animals in a va riety of ways. Some of them furnish food. furs or sport, others are interesting pets, but the great majority are injurious to agrkultural in terests. No less than 400 species and many additional sub-species have been described as occurring within the limits of North America, the great majority being mien The following is a modern classification of the order, through some mammalogists recog nize a larger number of families: Group Simplicidentata.,— There is a single pair of upper incisors with the enamel layer confined to the anterior surface; the fibula does not articulate with the calcaneum. Three sub orders: I. HysiricomsrPhs, or porcupine-like rodents: tibia and fibula distinct, alveolus of lower visor ending on the medial aide of the rams of the lower jaw, only one premolar in each jaw : Octodentide, containing the coypu and many others chiefly of South America, but a few in Africa; Hystricide, the porcupines,. Chinchillide, chinchillas of South America; Dasyerortide, agoutis, etc., of South America; Dfnoesyide, containing a single gen eralized rodent of Peru; Caviide, the cavies or guinea-pigs and eapybara of South America.
II. Sciowomorpha, or squirrel-like rodents: tibia and fibula distinct from each other, the alveolus of the lower incisor penetrating the ramus of the lower jaw and the premolars one or two on each side above and one on each side below. Families: Sciurida, the squirrels and
marmots, found throughout the world except Australia; iinonials. peculiar .squirrel-flike ale Wilk .1titegeher tiarat hetes along the side found in tropical Africa; Ha/dodo/aids, the sewellel or of the moun tains of the north Pacific coast; Castoride, the beavers of the northern hemisphere.
Myovvarphia, or rat-like rodents: tibia and fibula .coalesced, lower incisors as in the squirrels, premolar tot. Families: Dipodide, jumping mice, jerboas, etc. North America, Europe, Asia and Africa; Musa' the rats and mice, a very extensive and cosmopolitan variously subdivided; Myoride, the dorml Europe and other Old World forms; Geomyide, pouched gophers and pouched mice of America; Loehiomyide, a remarkable arboreal African form;Apolarida, the mole rats and their allies of the Old World.
Group Duolicidentata.— Besides the large incisors, the enamel covering of which extends around, there is a second Pair in the upper jaw; the fibula articulates, with the calcaneum and the ankle and elbow joints are toned and grooved; tibia and fibula co-ossified. One sob order : Lagonsattioa, bare-like rodents, with two Lepotide„ hares and rabbits, cosmo or whistling hares, North Ameriea and Europe. Fossil Rodentia—Rodent remains first oc cur in ,a fossil state in the Eocene Period, in which remains forms allied to the dormicq porcupine and squirrels exist. In Pliocene and post-Pliocene formations they become tolerably plentiful. Of the post-Tertiary forms, Trogos theriunk, or thereat beaver, found in European deposits, is a familiar species, and the cave beaver spelaus) is also a notable form. Trogontherium may possibly have survived the human period. The hares and rabbits first occur as fossils in Lower Miocene of North America and Pliocene of Europe, and the rats and mice are found in Eocene, Miocene and Pliocene formations. The guinea-pigs are chiefly Pleisto cene, their remains occurring in the Brazilian bone caves of that age. Some of the extinct rodents were as large as an ox. Very little light has been thrown on the origin of the order by a study of the fossil forms, though Professor Cope believed the Tillodonta to be their ances tors.
Bibliography.— Consult the articles in this work on the various animals included in the order, and the following: Cones and Allen, 'Monographs of North American Rodentia,' United States Geological Survey of Territories (Washington 1877) ; Elliot, 'Synopsis of the Mammals of North America,' Field Columbian Museum (Chicago 1901) ; Beddard, 'Mammals' (London 1902) ; Alton 'On the Classification of the Order Glires,' f'roc. Zoological Society (London 1876) ; Osborn, 'Age of Mammals' (New York 1910) numerous papers by Merriam, Miller, Palmer and others in the 'Bulletins' of the Biological Survey of the United States Department of Agriculture.