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FAMILY 112.—TRYGONIDÆ. (6 Genera, 43 Species.) “ Rays with the pectoral fins extending to end of snout."
DISTRIBUTION.—Seas of all temperate and tropical regions, and rivers of Tropical America. A species of Trygon has occurred on our Southern coast. Ellipesurus and Tæniura are found in the fresh waters of the interior of South America, while the latter genus occurs also in the Indian seas, but not in the Atlantic.
FAMILY 113.—MYLOBATIDÆ. (5 Genera, 22 Species.)
“ Rays with very broad pectoral fins not extending to end of snout.”
DISTRIBUTION.—Temperate and tropical seas. A species of Myliobatis is British, but most of the species and genera are confined to tropical seas. Dicerobatis and Ceratoptera are very large Rays, commonly called Sea-devils.
Cartilaginous fishes, with suctorial mouths and without lateral fins."
FAMILY 114.—PETROMYZONTIDÆ. (4 Genera, 12 Species.)
“ Marine or fresh-water eel-like fishes, with suctorial mouths and without barbels.”
DISTRIBUTION.-Coasts and fresh waters of temperate regions of both hemispheres. Three species of Petromyzon (Lampreys), are British
FAMILY 115.—MYXINIDÆ. (2 Genera, 5 Species.) “ Marine eel-like fishes, with four pairs of barbels.”
DISTRIBUTION.—Seas of the temperate regions of both hemispheres.
FAMILY 116.-CIRRHOSTOMI. (1 Genus, 1 Species.) “ A small marine fish with no jaws or fins, and with rudimentary eyes."
DISTRIBUTION.—The only species, the Lancelet (Amphioæus), is the lowest form of living vertebrate. It is found in the temperate regions of both hemispheres, and has occurred on our southern coast.
Remarks on the Distribution of Fishes. Marine Fish.—There are about 80 families of marine fishes, and of these no less than 50 are universally, or almost universally, distributed over the seas and oceans of the globe. Of the remainder many are widely distributed, some species even ranging from the North Atlantic to Australia. Six families are confined to the Northern Seas, but four of these consist of single species only, the other two being the Discoboli (2 genera, 11 sp.), and the Accipenseridæ (2 genera and 20 sp.). Only one family (Acanthoclinidæ) is confined to the Southern oceans, and that consists of but a single species. Four families (Sternoptychidæ)Stomiatidæ, Alepocephalidæ and Halosauridæ) are confined to the Atlantic Ocean, while 13 are found only in the Pacific; and of the remainder several are more abundant in the Pacific than the Atlantic. Two families (Lycodidæ and Gadidæ) are found in the Arctic and Antarctic seas only, though the
latter family has a single species in 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 Lophotidæ, found only in the Mediterranean and Japan; the small family of Notacanthi, confined to Greenland, the Mediterranean, and West Australia; and the four families, Sternoptychidæ, Stomiatidæ, Alepocephalidæ, and Halosauridæ, 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 peculiarities 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 Palæarctic has 12, and the Australian 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 shall find the fresh-water fishes in most cases markedly distinct. The Nearctic and Palæarctic 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
Trachinidæ possesses a fresh-water genus, Aphritis, one species of which inhabits Tasmania, and two others Patagonia; the Haplochitonidæ (2 genera, 3 sp.) are found only in Tierra del Fuego, the Falkland Islands, and South Australia; and the Galaxida (1 genus, 12 sp.) inhabit the same regions, but extend to Chili, to New Zealand and to Queensland. We have here an illustration of that connection between South America and Australia which is so strongly manifested in plants, but of which there are only scattered indications in most classes of animals. The dividing line across the Malay Archipelago, separating the Oriental from the Australian regions, and which is so strikingly marked in mammalia and birds, is equally so in fresh-water fishes. No less than six families have their eastern limits in Java and Borneo ; while the extensive family of Cyprinidæ has no less than 23 genera in Java and Borneo, but not a single species has been found in Celebes or the Moluccas.
The distribution of fresh-water fishes lends no support to the view that the peninsula of India belongs to the Ethiopian region. A large proportion of the Oriental families are common to the whole region; while there is hardly a single example, of a characteristic Ethiopian family or genus extending into the peninsula of India and no further.
Among the special peculiarities of distribution, is the curious fish, forming the family Comephoridæ, which is confined to Lake Baikal, among the mountains of Central Asia, 2,000 feet above the sea, and a thousand miles distant from the ocean; yet having its nearest allies in the exclusively oceanic family of the mackerels (Scomberidæ). The Characinidæ are confined to Africa and South America, distinct genera inhabiting each region. The Salmonidæ are confined to the two northern regions, except a single species of a peculiar genus in New Zealand. The genus Osteoglossum has a species in South America, another in the Sunda Islands, and a third in Queensland; while the curious Sirenoidei are represented by single species of peculiar genera in Tropical America, Tropical Africa, and Tropical Australia.
Fossil Fishes.—Fishes have existed from a very remote era, and it is remarkable that the first whose remains have been dis
covered belong to the Ganoidei, a highly developed group which has continued to exist down to our times, and of which the sturgeon is the best known example. We may therefore be sure that the Upper Silurian rocks in which these are found, although so very far back in geological history, do not by any means lead us to the time when the primitive fish-type appeared upon the earth. In the Carboniferous and Permian formations numerous remains of fishes are found, allied to the Lepidosteus or Gar-pike of North America. The next group in order of appearance, are the Plagiostomata, containing the existing Sharks and Rays. Traces of these are found in the highest Silurian beds, and be
plentiful in the Devonian and Carboniferous formations and in all succeeding ages, being especially abundant in Cretaceous and Eocene strata. The Holocephali appear first in the Oolitic period, and are represented by the living Chimæridæ. The Dipnoi, to which belong the Lepidosiren and Ceratodus, are believed to have existed in the Triassic period, from the evidence of teeth almost identical with those of the existing Australian fish. All the ancient fossil fishes belong to the above-mentioned groups, and many of them have little resemblance to existing forms. The Teleostean fishes, which form the great bulk of those now living, cannot be traced back further than the Cretaceous period, while by far the larger number first appear in the Tertiary beds. The Salmonidæ, Scopelidæ, Percidæ, Clupeidæ, Scombresocidæ, Mugilidæ, and Siluridæ, or forms closely allied to them, are found in the Cretaceous formation. In the Eocene beds we first meet with Squammipennes, Cyprinidæ, Pleuronectidæ, Characinida, Murænidæ, Gadidæ, Pediculati, Syngnathidæ, and Hippocampide.
Most of these fossils represent marine fishes, those of freshwater origin being rare, and of little importance as an aid in determining the causes of the distribution of living forms. To understand this we must look to the various changes of the land surface which have led to the existing distribution of all the higher vertebrates, and to those special means of dispersal which Mr. Darwin has shown to be possessed by all fresh-water productions.