The Muridæ, comprising the rats and mice with their allies, are almost universally distributed over the globe (even not reckoning the domestic species which have been introduced almost everywhere by man), the exceptions being the three insular groups belonging to the Australian region, from none of which have any species yet been obtained. Before enumerating the genera it will be as well to say a few words on the peculiarities of distribution they present. The true mice, forming the genus _Mus_, is distributed over the whole of the world except N. and S. America where not a single indigenous species occurs, being replaced by the genus _Hesperomys_; five other genera, comprehending all the remaining species found in South America are peculiar to the Neotropical region. Three genera are confined to the Palæarctic region, and three others to the Nearctic. No less than twelve genera are exclusively Ethiopian, while only three are exclusively Oriental and three Australian. _Mus_ (100-120 sp.) the Eastern Hemisphere, but absent from the Pacific and Austro-Malayan Islands, except Celebes and Papua; _Lasiomys_ (1 sp.) Guinea; _Acanthomys_ (5-6 sp.) Africa, India and {230}N. Australia; _Cricetomys_ (1 sp.) Tropical Africa; _Saccostomus_ (2 sp.) Mozambique; _Cricetus_ (9 sp.) Palæarctic region and Egypt; _Cricetulus_ (1 sp., Milne-Edwards, 1870) Pekin; _Pseudomys_ (1 sp.) Australia; _Hapalotis_ (13 sp.) Australia; _Phlæomys_ (1 sp.) Philippines; _Platacanthomys_ (1 sp., Blyth, 1865) Malabar; _Dendromys_ (2 sp.) S. Africa; _Nesomys_ (1 sp. Peters, 1870) Madagascar; _Steatomys_ (2 sp.) N. and S. Africa; _Pelomys_ (1 sp.) Mozambique; _Reithrodon_ (9 sp.) N. America, Lat. 29° to Mexico, and south to Tierra del Fuego; _Acodon_ (1 sp.) Peru; _Myxomys_ (1 sp.) Guatemala; _Hesperomys_ (90 sp.) North and South America; _Holochilus_ (4 sp.) South America; _Oxymycterus_ (4 sp.) Brazil and La Plata; _Neotoma_ (6 sp.) U.S., East coast to California; _Sigmodon_ (2 sp.) Southern United States; _Drymomys_ (1 sp.) Peru; _Neotomys_ (2 sp.) S. America; _Otomys_ (6 sp.) S. and E. Africa; _Meriones_ = _Gerbillus_ (20-30 sp.) Egypt, Central Asia, India, Africa; _Rhombomys_ (6 sp.) S. E. Europe, N. Africa, Central Asia; _Malacothrix_ (2 sp.) South Africa; _Mystromys_ (1 sp.) South Africa; _Psammomys_ (1 sp.) Egypt; _Spalacomys_ (1 sp.) India; _Sminthus_ (1-3 sp.) East Europe, Tartary, Siberia; _Hydromys_ (5 sp.) Australia and Tasmania; _Hypogeomys_ (1 sp., Grandidier, 1870) Madagascar; _Brachytarsomys_ (1 sp., Günther, 1874) Madagascar; _Fiber_ (2 sp.) N. America to Mexico; _Arvicola_ (50 sp.) Europe to Asia Minor, North Asia, Himalayas, Temp. N. America; _Cuniculus_ (1 sp.) N. E. Europe, Siberia, Greenland, Arctic America; _Myodes_ (4 sp.) Europe, Siberia, Arctic America, and Northern United States; _Myospalax_ = _Siphneus_ (2 sp.) Altai Mountains and N. China[4]; _Lophiomys_ (1 sp.) S. Arabia, and N. E. Africa; _Echiothrix_ (1 sp.) Australia.