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CHAP. XX.] FISHES. 465
latter family has a single species iu the Indian seas. Among the
curiosities of distribution are,—the extensive genus Diagramma,
confined to the Pacific with the exception of one species in the
Mediterranean ;
the single species constituting the family Lopho-
tidee, found only in the Mediterranean and Japan ; the small
family of Notacanthi, confined to Greenland, the Mediter-
ranean, and West Australia; and the four families, Sternop-
tychidse, Stomiatidse, Alepocephalidse, and Halosauridae, which
are believed to inhabit exclusively the depths of the ocean, and
are therefore very rarely obtained.
Fresh-water Fish.—There are 36 families of fishes which
inhabit fresh water exclusively, and 5 others, which are both
marine and fresh-water. These present many interesting pecu-
liarities of distribution. The Neotropical region is the richest
in families, and probably also in genera and species. No less
than 22 families inhabit it, and of these 6 are altogether peculiar.
The Ethiopian and Nearctic regions each have 18 families, the
former with 3, and the latter with 5 peculiar. Several isolated
forms, requiring to be placed in distinct families, inhabit the
great American lakes ; and, no doubt, when the African lakes
are equally well known, they will be found also to possess many
peculiar forms. The Oriental region comes next, with 17 families,
of which 3 are peculiar. The Palaearctic has 12, and the Aus-
tralian 11 families, each with only 1 altogether peculiar to it.
If we take those regions which are sometimes supposed to be
so nearly related that they should be combined, we shaU find the
fresh-water fishes in most cases markedly distinct. The Nearctic
and Palaearctic regions, for example, together contain 20 families,
but only 11 of these occur in both, and only 5 are exclusive
inhabitants of these two regions. This shows an amount of
diversity that would not, perhaps, be exhibited by any other
class of animals. The Ethiopian and Oriental regions together
possess 24 families, only 11 of which are found in both, and
only 1 exclusively characteristic of the two. The Australian
and Neotropical regions possess together 27 families, of which 7
are found in both, and 3 are exclusively characteristic of the
two. This last fact is very interesting : the marine family of