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1. The Order Rodentia is the most numerous of all the chief Mammalian groups, containing nearly 1400 species, which are divided into 170 genera and 21 families.
2. The Rodents are distributed all over the world both in temperate and tropical climes, and are abundant everywhere, both in species and individuals, except in Australia where they are poorly represented.
3. The most all-pervading and numerous family of the Rodents is the Muridm (Mice), which are upwards of 600 in number and are the only members of the group met with in Australia and Madagascar. In Madagascar the Muridm are represented by a small group of indigenous genera.
4. The Porcupine-like Rodents, of which there are seven families, are specially characteristic of the New World, only the typical Porcupines (Hystricidm) and five peculiar genera of Octodonts being inhabitants of the New World.
5. Two families of Rodents, the Pocket-gophers (Geomyidm) and the Kangaroo-rats (Heteroinyidm), are specially characteristic of the Nearctic Region, the Jerboas (Dipodidm) of the Palsearctic, and the Sand-rats (Bathyergidm) of the Ethiopian Region.