characters of the equatorial zone, has received some characteristic Japanese freshwater fishes. Within the geographical boundaries of China the freshwater fishes of the tropics pass gradually into those of the northern zone, both being separated by a broad, debateable ground. The affluents of the great river traversing this district are more numerous from the south than from the north, and carry the southern fishes far into the temperate zone. Scarcely better defined is the boundary of this region towards the north-west, in which fishes were very poorly represented by types common to India and Africa.
The African region comprises the whole of Africa south of the Sahara. It might have been conjectured that the more temperate climate of its southern extremity would have been accompanied by a conspicuous difference in the fish fauna. But this is not the case; the difference between the tropical and southern parts of Africa consists simply in the gradual disappearance of specifically tropical forms, whilst Silurids, Cyprinids and even Anabas penetrate to its southern coast; no new form, except a Galaxias at the Cape of Good Hope, has entered to impart to South Africa a character distinct from the central portion of the continent. In the north-east the African fauna passes the isthmus of Suez and penetrates into Syria; the system of the Jordan presents so many African types that it has to be included in a description of the African region as well as of the Europaeo-Asiatic.
The boundaries of the Neotropical or Tropical American region have been sufficiently indicated in the definition of the equatorial zone. A broad and most irregular band of country, in which the South and North American forms are mixed, exists in the north.
The Tropical Pacific region includes all the islands east of Wallace’s line, New Guinea, Australia (with the exception of its south-eastern portion), and all the islands of the tropical Pacific to the Sandwich group.
Northern Zone.—The boundaries of the northern zone coincide in the main with the northern limit of the equatorial zone; but they overlap the latter at different points. This happens in Syria, as well as east of it, where the mixed faunae of the Jordan and the rivers of Mesopotamia demand the inclusion of this territory in the northern zone as well as in the equatorial; in the island of Formosa, where a Salmonid and several Japanese Cyprinids flourish; and in Central America, where a Lepidosteus, a Cyprinid (Sclerognathus meridionalis), and an Amiurus (A. meridionalis) represent the North American fauna in the midst of a host of tropical forms.
There is no separate arctic zone for freshwater fishes; ichthyic life becomes extinct towards the pole wherever the fresh water remains frozen throughout the year, or thaws for a few weeks only; and the few fishes which extend into high latitudes belong to types in no wise differing from those of the more temperate south. The highest latitude at which fishes have been obtained is 82° N. lat., whence specimens of char (Salmo arcturus and Salmo naresii) have been brought back.
The Palaearctic or Europaeo-Asiatic Region.—The western and southern boundaries of this region coincide with those of the northern zone. Bering Strait and the Kamchatka Sea have been conventionally taken as the boundary in the north, but the fishes of both coasts, so far as they are known, are not sufficiently distinct to be referred to two different regions. The Japanese islands exhibit a decided Palaearctic fish fauna with a slight influx of tropical forms in the south. In the east, as well as in the west, the distinction between the Europaeo-Asiatic and the North American regions disappears almost entirely as we advance farther towards the north. Finally, the Europaeo-Asiatic fauna mingles with African and Indian forms in Syria, Persia and Afghanistan.
The boundaries of the North American or Nearctic region have been sufficiently indicated. The main features and the distribution of this fauna are identical with those of the preceding region.
Southern Zone.—The boundaries of this zone have been indicated in the description of the equatorial zone; they overlap the southern boundaries of the latter in South Australia and South America, but we have not the means of defining the limits to which southern types extend northwards. This zone includes Tasmania, with at least a portion of south-eastern Australia (Tasmanian sub-region), New Zealand and the Auckland Islands (New Zealand sub-region), and Chile, Patagonia, Tierra del Fuego and the Falkland Islands (Fuegian sub-region). No freshwater fishes are known from Kerguelen’s Land, or from islands beyond 55° S. lat.
The Tropical American region is the richest (about 1300 species); next follow the African region (about 1000), the Indian region (about 800), the Europaeo-Asiatic region (about 500), the North American region (about 400), the Tropical Pacific region (about 60); whilst the Antarctic region is quite insignificant.
Of the migratory fishes, or fishes travelling regularly from the sea to fresh waters, most, if not all, were derived from marine forms. The anadromous forms, annually or periodically ascending rivers for the purpose of spawning, such as several species of Acipenser, Salmo, Coregonus, Clupea (shads), and Petromyzon, are only known from the northern hemisphere, whilst the catadromous forms, spending most of their life in fresh water but resorting to the sea to breed, such as Anguilla, some species of Mugil, Galaxias and Pleuronectes, have representatives in both hemispheres. (G. A. B.)
ICHTHYOPHAGI (Gr. for “fish-eaters”), the name given
by ancient geographers to several coast-dwelling peoples in
different parts of the world and ethnically unrelated. Nearchus
mentions such a race as inhabiting the barren shores of the
Mekran on the Arabian Sea; Pausanias locates them on the
western coast of the Red Sea. Ptolemy speaks of fish-eaters
in Ethiopia, and on the west coast of Africa; while Pliny
relates the existence of such tribes on the islands in the Persian
Gulf. Herodotus (book i. c. 200) mentions three tribes of the
Babylonians who were solely fish-eaters, and in book iii. c. 19
refers to Ichthyophagi in Egypt. The existence of such tribes
was confirmed by Sir Richard F. Burton (El-Medinah, p. 144).
ICHTHYOSAURUS, a fish or porpoise-shaped marine reptile which characterized the Mesozoic period and became extinct immediately after the deposition of the Chalk. It was named Ichthyosaurus (Gr. fish-lizard) by C. König in 1818 in allusion to its outward form, and is best known by nearly complete skeletons from the Lias of England and Germany. The large head is produced into a slender, pointed snout; and the jaws are provided with a row of conical teeth nearly uniform in size and deeply implanted in a continuous groove. The eye is enormous, and is surrounded by a ring of overlapping “sclerotic plates,” which would serve to protect the eye-ball during diving. The vertebrae are very numerous, short and deeply biconcave, imparting great flexibility to the backbone as in fishes. The neck is so short and thick that it is practically absent. There are always two pairs of paddle-like limbs, the hinder pair never disappearing as in porpoises and other Cetacea, though often much reduced in size. A few specimens from the Upper Lias of Württemberg (in the museums of Stuttgart, Tübingen, Budapest and Chicago) exhibit remains of the skin, which is quite smooth and forms two triangular median fins, one in the middle of the back, the other at the end of the tail. The dorsal fin consists merely of skin without any internal skeleton, while
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| From British Museum Guide to Fossil Reptiles and Fishes, by permission of the Trustees. |
| Skeleton of Ichthyosaurus communis, with outline of body and fins, from the Lower Lias of Lyme Regis, Dorset; original nearly four metres in length. |
