_Fresh-water Fishes._--The Nearctic region possesses no less than five peculiar family types, and twenty-four peculiar genera of this class. The families are Aphredoderidæ, consisting of a single species found in the Eastern States; Percopsidæ, founded on a species peculiar to Lake Superior; Heteropygii, containing two genera peculiar to the Eastern States; Hyodontidæ and Amiidæ, each consisting of a single species. The genera are as follows: _Paralabrax_, found in California; _Huro_, peculiar to Lake Huron; _Pileoma_, _Boleosoma_, _Bryttus_ and _Pomotis_ in the Eastern States--all belonging to the perch family. _Hypodelus_ and _Noturus_, belonging to the Siluridæ. _Thaleichthys_, one of the Salmonidæ peculiar to the Columbia river. _Moxostoma_, _Pimephales_, _Hyborhynchus_, _Rhinichthys_, in the Eastern States; _Ericymba_, _Exoglossum_, _Leucosomus_, and _Carpiodes_, more widely distributed; _Cochlognathus_, in Texas; _Mylaphorodon_ and _Orthodon_, in California; _Meda_, in the river Gila; and _Acrochilus_, in the Columbia river--all belonging to the Cyprinidæ. _Scaphirhynchus_, found only in the Mississippi and its tributaries, belongs to the sturgeon family (Accipenseridæ). _Summary of Nearctic Vertebrata._--The Nearctic region possesses 24 peculiar genera of mammalia, 49 of birds, 21 of reptiles, and 29 of fresh-water fishes, making 123 in all. Of these 70 are mammals and land-birds, out of a total of 242 {121}genera of these groups, a proportion of about two-sevenths. This is the smallest proportion of peculiar genera we have found in any of the regions; but many of the genera are of such isolated and exceptional forms that they constitute separate families, so that we have no less than 12 families of vertebrata confined to the region. The Palæarctic region has only 3 peculiar families, and even the Oriental region only 12; so that, judged by this test, the Nearctic region is remarkably well characterized. We must also remember that, owing to the migration of many of its peculiar forms during the Glacial period, it has recently lost some of its speciality; and we should therefore give some weight to the many characteristic groups it possesses, which, though not quite peculiar to it, form important features in its fauna, and help to separate it from the other regions with which it has been thought to be closely allied. It is thus well distinguished from the Palæarctic region by its Procyonidæ, or racoons, _Hesperomys_, or vesper mice, and _Didelphys_, or opossums, among Mammalia; by its Vireonidæ, or greenlets, Mniotiltidæ, or wood-warblers, Icteridæ, or hang-nests, Tyrannidæ, or tyrant shrikes, and Trochilidæ, or humming-birds, among birds, families which, extending to its extreme northern limits must be held to be as truly characteristic of it as of the Neotropical region; by its Teidæ, Iguanidæ, and _Cinosternum_, among reptiles; and by its Siluridæ, and Lepidosteidæ, among fishes. From