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The Hydrophidae, or Sea Snakes, are a group of small-sized marine Serpents, abundant in the Indian and Australian seas, and extending as far west as Madagascar, and as far east as Panama. They are very poisonous, and it is probable that many species remain to be discovered. The genera are distributed as follows:— - - Hydrophis (37 sp.), ranging from India to Formosa and Australia; Platurus (2 sp.), from the Bay of Bengal to New Guinea and New Zealand; Aipysurus (3 sp.), Java to New Guinea and Australia; Disteira (1 sp.), unknown locality; Acalyptus (1 sp.), South-west Pacific ; Enhydrina (1 sp.), Bay of Bengal to New Guinea; Pelamis (1 sp.), Madagascar to New Guinea, New Zealand, and Panama; Emydocephalus (1 sp.), Australian Seas.

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The Crotalidae, or Pit Vipers, including the deadly Rattlesnakes, form a well-marked family of fanged serpents, whose distribution is very interesting. They abound most in the Oriental region, at least 5 of the genera and 20 species being found within its limits, yet they are quite unknown in the Ethiopian region —a parallel case to that of the Bears and Deer. A few species are peculiar to the eastern portion of the Palaearctic region, while the Nearctic is actually richer than the Neotropical region both in genera and species. This would point to the conclusion, that the group originated in the Indo-Chinese sub-region and spread thence north-east to North America, and so onward to South America, which, having been the last to receive the group, has not had time to develop it largely, notwithstanding its extreme adaptability to Reptilian life. The genera are divided among the several regions as follows:— Craspedocephalus (7 sp.), Tropical America and the West Indian Islands; Cenchris, Crotalophorus, Uropsophorus, and Crotalus, inhabiting North America from Canada and British Columbia to Texas, one species (Crotalus horridus) extending into South America; Trimeresurus (16 sp.), all India from Ceylon to Assam, Eormosa, the Philippines and Celebes; Peltopelor and Hypnale (1 sp. each), peculiar to India ; Calloselasma (1 sp.), Siam ; Atropos (1 sp.), Java and Borneo; Halys (3 sp.), peculiar to Tartary, Thibet, Japan, North China, and Formosa.

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The Viperidae, or True Vipers, are especially characteristic of the Palaearctic and Ethiopian regions, only one species being found over a large part of the Oriental region, and another reaching Central India. They are especially abundant in Africa, and the Palaearctic confines in South-western Asia. The common Viper ranges across the whole Palaearctic region from Portugal to Saghalien Island, reaching to 67° North Latitude, in Scandinavia, and to 58° in Central Siberia. The genera, according to Dr. Strauch's synopsis, are distributed as follows:—

Vipera (17 sp.), which has the range of the family, extending over the whole of the Palaearctic and Ethiopian regions, except Madagascar, and as far as Ceylon, Siam, and Java, in the Oriental region; Echis (2 sp.), inhabiting North Africa to Persia and to Continental India ; and Atheris (3 sp.), confined to West Africa.

Remarks on the General Distribution of Ophidia.

The Ophidia, being preeminently a Tropical order—rapidly diminishing in numbers as we go north in the Temperate Zone, and wholly ceasing long before we reach the Arctic Circle—we cannot expect the two Northern regions to exhibit any great variety or peculiarity. Yet in their warmer portions, they are tolerably rich; for, of the 25 families of snakes, 6 are found in the Nearctic region, 10 in the Palaearctic, 13 in the Australian, 16 in the Neotropical, 17 in the Ethiopian, and no less than 22 in the Oriental, which last is thus seen to be by far the richest of the great regions in the variety of its forms of Ophidian life. The only regions that possess altogether peculiar families of this order, are the Ethiopian (3), and the Oriental (2); the usually rich and peculiar Neotropical region not possessing exclusively, any family of snakes; and what is still more remarkable, the Neotropical and Australian regions together, do not possess a family peculiar to them. Every family inhabiting these two regions is found also in the Oriental; and this fact, taken in connection with the superior richness of the latter region both in families and genera, would indicate that the Ophidia had their origin in the northern hemisphere of the Old World (the ancient Palaearctic region) whence they spread on all sides, in successive waves of migration, to the other regions. The distribution of the genera peculiar to, or highly characteristic of, the several regions is as follows:—

The Nearctic possesses 9 ; four of these belong to the Colubridae, one to the Pythonidae, and four to the Crotalidae. The Palaearctic region has only 2 peculiar genera, belonging to the Colubridae and Crotalidae. The Ethiopian has 25, belonging to 11 families; four to Colubridae, five to Lycodontidae, and three to Elapidae. The Oriental has no less than 50, belonging to 15 families; five are Colubridae, five Uropeltidae, twelve Homalopsidae, six Lycodontidae, three Amblycephalidae, eight Elapidae, and four Crotalidae. The Australian has 16, belonging to three families only; eleven being Elapidae, and four Pythonidae. The Neotropical has about 24, belonging to eight families; ten are Colubridae, six Pythonidae, and the rest Dipsadidae, Scytalidae, Amblycephalidae, Elapidae, and Crotalidae. We find then, that in the Ophidia, the regions adopted in this work are remarkably distinct; and that, in the case of the Oriental and Ethiopian, the difference is strongly marked, a very large number of the genera being confined to each region. It is interesting to observe, that in many cases the affinity seems to be rather between the West Coast of Africa and the Oriental region, than between the East Coast and the plains of India; thus the Homalopsidae—a highly characteristic Oriental family— occur on the West Coast of Africa only; the Dryiophidae, which range over the whole Oriental region, only occur in Madagascar and West Africa in the Ethiopian; the genus Dipsas is found over all the Oriental region and again in West Africa. A cause for this peculiarity has been suggested in our sketch of the past history of the Ethiopian region, Wol. I. p. 288. In the Lycodontidae, which are strictly confined to these two regions, the genera are all distinct, and the same is the case with the more widely distributed Elapidae; and although a few desert forms, such as Echis and the Erycidae, are common to Africa and the dry plains of India, this is evidently due to favourable climatic conditions, and cannot neutralise the striking differences in the great mass of the family and generic forms which inhabit the two regions. The union of Madagascar with the South-western part of the Oriental region under the appellation Lemuria, finds no support in the distribution of Ophidia; which, however, strikingly accords with the views developed in the Third Part of this work, as to the great importance and high antiquity of the Euro-Asiatic contiment, as the chief land-centre from which the higher organisms have spread over the globe. Fossil Ophidia.-The oldest known remains of Ophidia occur in the Eocene formation in the Isle of Sheppey; others are found in the Miocene (Brown Coal) of Germany, and in some Tertiary beds in the United States. Most of these appear to have been large species belonging to the Pythonidae, so that we are evidently still very far from knowing anything of the earliest forms of this order. In some of the later Tertiary deposits the poison fangs of venomous species have been found; also a Colubrine snake from the Upper Miocene of the South of France.

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The single species of Trogonophis, forming this family, is found only in North Africa.

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Chirotes, the genus which constitutes this family, inhabits Mexico, and has also been found in Missouri, one of the Southern United States.

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