As boundaries within which this fauna is comprised, may be indicated 30° lat. N. and S., as in the Indo-Pacific. Its distinction from the Indo-Pacific lies in the almost entire absence of coral-feeding fishes. There are scarcely any Squamipinnes, Pharyngognaths or Acronuridæ, and the Teuthyes are entirely absent. The genera that remain are such as are found in the tropical zone generally, but the species are entirely different from those of the Indo-Pacific. They are mixed with a sprinkling of peculiar genera, consisting of one or two species, like Discopyge, Hoplopagrus, Doydixodon, but they are too few in number to give a strikingly peculiar character to this fauna.
The Three districts are distinguishable:—
a. Central American district, in which we include, for the present, Lower California, shows so near an affinity to the tropical Atlantic that, if it were not separated from it by the neck of land uniting the two American Continents, it would most assuredly be regarded as a portion of the Fauna of the tropical Atlantic. With scarcely any exceptions the genera are identical, and of the species found on the Pacific side nearly one-half have proved to be the same as those of the Atlantic. The explanation of this fact has been found in the existence of communications between the two oceans by channels and straits which must have been open till within a recent period. The isthmus of Central America was then partially submerged, and appeared as a chain of islands similar to that of the Antilles; but as the reef-building corals flourished chiefly north and east of those islands, and were absent south and west of them, reef-fishes were excluded from the Pacific shores when the communications were destroyed by the upheaval of the land.
b. The Galapagoes district received its coast fauna principally from the Central American district, a part of the species being absolutely the same as on the coast of the Isthmus of Panama, or as in the West Indies. Yet the isolation of this group has continued a sufficiently long period to allow of the development of a number of distinct species of either peculiarly Atlantic genera (such as Centropristis, Rhypticus, Gobiesox, Prionotus), or at least tropical genera (such as Chrysophrys, Pristipoma, Holacanthus, Caranx, Balistes). A few other types from the Peruvian coast (Doydixodon), or even from Japan (Prionurus), have established themselves in this group of islands. A species of Cestracion has also reached the Galapagoes, but whether from the south, north, or west, cannot be determined.
The presence of the Atlantic fauna on the Pacific side is felt still farther west than the Galapagoes, some Atlantic species having reached the Sandwich Islands, as Chætodon humeralis and Blennius brevipinnis.
c. The Peruvian district possesses a very limited variety of shore fishes, which belong, with few exceptions, like Discopyge, Hoplognathus, Doydixodon, to genera distributed throughout the tropical zone, or even beyond it. But the species, so far as they are known at present, are distinct from those of the Indo-Pacific, as well as of the tropical Atlantic; and therefore this district cannot be joined either to the Central American or the Galapagoes.
IV.—The Southern Temperate Zone.
This zone includes the coasts of the southern extremity of Africa, from about 30° lat. S., of the south of Australia with Tasmania, of New Zealand, and the Pacific and Atlantic coasts of South America between 30° and 50° lat. S.
The most striking character of this fauna is the reappearance of types inhabiting the corresponding latitudes of the Northern Hemisphere, and not found in the intervening tropical zone. This interruption of the continuity in the geographical distribution of Shore-fishes is exemplified by species as well as genera, for instance—Chimæra monstrosa, Galeus canis, Acanthias vulgaris, Acanthias blainvillii, Rhina squatina, Zeus faber, Lophius piscatorius, Centriscus scolopax, Engraulis encrasicholus, Clupea sprattus, Conger vulgaris. Instances of genera are still more numerous—Cestracion, Spinax, Pristiophorus, Raja; Callanthias, Polyprion, Histiopterus, Cantharus, Box, Girella, Pagellus, Chilodactylus, Sebastes, Aploactis, Agonus, Lepidopus, Cyttus, Psychrolutidæ, Notacanthus; Lycodes, Merluccius, Lotella, Phycis, Motella; Aulopus; Urocampus, Solenognathus; Myxine.
Naturally, where the coasts of the tropical zone are continuous with those of the temperate, a number of tropical genera enter the latter, and genera which we have found between the tropics as well as in the temperate zone of the Northern Hemisphere, extend in a similar manner towards the south. But the truly tropical forms are absent; there are no Squamipinnes, scarcely any Mullidæ, no Acronuri, no Teuthyes, no Pomacentridæ (with a single exception on the coast of Chili), only one genus of Julidina, no Scarina, which are replaced by another group of Pharyngognaths, the Odacina. The Labrina, so characteristic of the temperate zone of the Northern Hemisphere, reappear in a distinct genus (Malacopterus) on the coast of Juan Fernandez.