In the Madrepora tribe, all of the Madreporids and Poritids, many of the Dendrophylla family, or Eupasamnaids.
Among Alcyonoids, numerous species of the Alcyonum and Gorgonia, and some of the Pen natulacea.
Among Hydroids, the Millepores and Heliopores.
Among Algm, many Nullipores and Corallines.
The corals of colder waters aro mostly solitary polyys, either outside the coral reef seas, or at considerable depths:within them, and comprise a few Fungids, some Oculinids, many Eupsammids, some of the Gorgona and Pennatula tribes, and a few of the Alcyonium tribe ; a few Milleporids of the genus Pleobothros.
Coral Reefs are classed by most authors into lagoon islands or atolls, barrier or encircling reefs, and fringing or shore reefs.
The Atoll or Lagoon Reefs are vast rings of coral rock, often many leagues in diameter, here and there surmounted by a low verdant island, with dazzling white shoroi, bathed on the outside by the foaming breakers of the ocean, and on the inside surrounding a calm expanse of water, which, from reflection, is generally of a bright but pale green colour. These are raised by soft and almoat gelatinous coral polypifers, which work on increasing the outer edge of the reef, which day and night is lashed by the breakers of an ocean never at rest.
Barrierl?eefs are little lessmarvellous than atolls. In rare places, the whole of the part of tho reef that is visible is converted into land ; but more usually tile barrier reef is shown by a snow-white line of breakers, with hero and there an islet crowned by cocoanut trees, and separating the smooth waters of the lagoon-like channel from the waves of the open sea. There are many such outside the small islands in the Pacific Ocean, but the 13arrier Reefs of Australia and New Caledonia have excited nluch attention from their great size.
Fringing Reefs or Shore Reefs differ from barrier reefs in not lying far off shore, and in not having within them a broad channel of deep water.
Darwin supposes that as the reef-building polypifers can flourish only at limited depths, the foundation on which the coral was prirnarily attached has subsided, alike in the case of the atoll as in that of the barrier reef.
Sometimes the barrier reef recedes from the shore, and forms wide channels or inland seas, where ships find ample room and depth of water, exposed, however, to the danger of hidden reefs. The reef on the north-east coast of Anstralia and New Caledonia extends 400 miles, at a distance varying from 30 to GO miles from the shore, and having many fathoms of depth in the channel. West of the large Fiji Islands, the channel is in sotne parts 23 miles wide, and twelve to forty fathoms in depth. The sloop-of-war Peacock sailed along the west coast of both Viti Lebu and Vanna Lebu, within the inner reefs, a distance exceeding 200 miles. A barrier reef, enclosing a lag,00n, is the general formation of the coral islands, though there are some of small size in which the lagoon is wanting. These are found in all stages of development ; in sotne the reef is narrow and broken, forming a succession of narrow islets with openings into the lagoon ; in others there only remains a depression of surface in the centre to indicate where the lagoon origin ally was. The most beautiful are those where the lag0011 i8 completely enclosed, and rests within a quiet lake. Maraki, ono of the Kingsmill group, is one of the prettiest coral islands of the Pacific. The lino of vegetation is unbroken, and, seen from the mast-head, it lies like a garland thrown upon the waters. It is in the South Pacific Ocean that coml reefs and coral island.s are seen in the greatest perfection. The largest known coral reef is the Great Barrier Reef, that runs for 1000 miles parallel to the coast of Austmlia, and at a distance from the shore of from 20 to GO miles. The barrier reef of New Caledonia is 40 miles long.—Dana, Corals and Coral Islands ; Darwin, Naturalises Voyage ; Darwin, Structure of Coral Reefs ; Gosse, Natural History ; 3fac gillivray's Voyage; Figuier, Ocean World; .3/Oury, Physical Geog. ; Hartwig ; Jansen ; Collingwood.