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ANTHOZOA

and the solvent action of sea-water. Barrier reefs may be regarded as fringing reefs on a large scale. The great Australian barrier reef extends for no less a distance than 1250 miles from Torres Strait in 9-5° S. lat. to Lady Elliot island in 24° S. lat. The outer edge of a barrier reef is much farther from the shore than that of a fringing reef, and the channel between it and the land is much deeper. Opposite Cape York the seaward edge of the great Australian barrier reef is nearly 90 miles distant from the coast, and the maximum depth of the channel at this point is nearly 20 fathoms. As is the case in a fringing reef, the outer edge of a barrier reef is in many places awash at low tides, and masses of dead coral and sand may be piled up on it by the action of the waves, so that islets are formed which in time are covered with vegetation. These islets may coalesce and form a strip of dry land lying some hundred yards or less from the extreme outer edge of the reef, and separated by a wide channel from the mainland. Where the barrier reef is not far from the land there are always gaps in it opposite the mouths of rivers or considerable streams. The outer wall of a barrier reef is steep, and frequently, though not always, descends abruptly into great depths. In many cases in the Pacific Ocean a barrier reef surrounds one or more island peaks, and the strip of land on the edge of the reef may encircle the peaks with a nearly complete ring. An atoll is a ring-shaped reef, either awash at low tide or surmounted by several islets, or more rarely by a complete strip of dry land surrounding a central lagoon. The outer wall of an atoll generally descends with a very steep but irregular slope to a depth of 500 fathoms or more, but the lagoon is seldom more than 20 fathoms Coral formations. deep, and may be much less. Frequently, especially to Many species of corals are widely distributed, and are the leeward side of an atoll, there may be one or more found at all depths both in warmer and colder seas. navigable passages leading from the lagoon to the open Lophohelia prolifera and DendrophyUia ramea form sea. Though corals flourish everywhere under suitable condense beds at a depth of from 100 to 200 fathoms off the coasts of Norway, Scotland, and Portugal, and the ditions in tropical seas, coral reefs and atolls are by no Challenger and other deep-sea dredging expeditions have means universal in the torrid zone. The Atlantic Ocean brought up corals from great depths in the Pacific and is remarkably free from coral formations, though there are Atlantic oceans. But the larger number of species, par- numerous reefs in the West Indian islands, off the south ticularly the more massive kinds, occur only in tropical coast of Florida, and on the coast of Brazil. The Bermudas seas in shallow waters, whose mean temperature does not also are coral formations, their high land being formed fall below 68° Fahr., and they do not flourish unless the by sand accumulated by the wind and cemented into rock, temperature is considerably higher. These conditions of and are remarkable for being the farthest removed from temperature are found in a belt of ocean which may the equator of any recent reefs, being situated in 32° N. roughly be indicated as lying between the 28th N. and lat. In the Pacific Ocean there is a vast area thickly S. parallels. Within these limits there are numerous dotted with coral formations, extending from 5° N. lat. reefs and islands formed of coral intermixed with the cal- to 25° S. lat., and from 130° E. long, to 145° W. long. careous skeletons of other animals, and their formation There are also extensive reefs in the westernmost islands has long been a matter of dispute among naturalists and of the Hawaiian group in about 25° N. lat. In the geologists. Indian Ocean, the Laccadive and Maidive islands are Coral formations may be classed as fringing or shore large groups of atolls off the west and south-west of reefs, barrier reefs, and atolls. Fringing reefs are plat- India. Still farther south is the Chagos group of atolls, forms of coral rock extending no great distance from the and there are numerous reefs off the north coast of shores of a continent or island. The seaward edge of the Madagascar, at Mauritius, Bourbon, and the Seychelles. platform is usually somewhat higher than the inner part, The Cocos-Keeling Islands, in 12° S. lat. and 96° E., and is often awash at low water. It is intersected by long., are typical atolls in the eastern part of the Indian numerous creeks and channels, especially opposite those Ocean. places where streams of fresh water flow down from the The remarkable characters of barrier reefs and atolls, land, and there is usually a channel deep enough to be their isolated position in the midst of the great oceans, navigable by small boats between the edge of the reef and the seemingly unfathomable depths from which they rise, the land. The outer wall of the reef is rather steep, but their peaceful and shallow lagoons and inner channels, descends into a comparatively shallow sea. Since corals their narrow strips of land covered with cocoa-nut palms are killed by fresh water or by deposition of mud or sand, and other vegetation, and rising but a few feet above the it is obvious that the outer edge of the reef is the region level of the ocean, naturally attracted the attention of the of most active coral growth, and the boat channel and the earlier navigators, who formed sundry speculations as to passages leading into it from the open sea have been their origin. The poet-naturalist Chamisso was the first formed by the suppression of coral growth by one of the to propound a definite theory of the origin of atolls and above-mentioned causes, assisted by the scour of the tides encircling reefs, attributing their peculiar features to the

/Family 9. Cyathophylt.id^.—Solitary and colonial aporose corals. Tabnlse and vesicular endotheca. present. Septa numerous, generally radial, seldom pinnate. Typical genera—Cyathophyllum, Goldfuss (Devonian and Carboniferous). Moseleya, Quelch (recent). _ Family 10. Astramdu;. — Aporose, mainly colonial corals, massive, branching, or mseandroid. Septa radial; dissepiments present; an epitheca surrounds the base of massive or nucandroid forms, but only surrounds individual corallites in simple or branching forms. Typical genera C5 ctstTcccc, M. Edw. and H. Heliustvccct) M. Edw. and H. Moeandrina, Lam. Cmloria, M. Edw. and H. Favia, Oken. Family 11. Fungid,®. — Solitary and colonial corals, with numerous radial septa united by synapticuke. Typical genera—Lophoscvis, M. Edw. and H. ThcimTMstrQM, Le Sauvage. Leptophyllia, Reuss (Jurassic and Cretaceous). I Fungia, Dana. Siclemstrcea, Blainv. f Family 12. Eupsammida:. — Solitary or colonial perforate corals, branching, massive, or encrusting. Septa radial; the primary septa usually compact, the remainder perforate. Theca perforate. Synapticula present in some genera. Typical genera—Stepimnophyllia, Michelin. Eupsammia, O M. Edw. and H. Astroides, Blainv. Bhodopsammia, M. I Edw. and H. DendrophyUia, M. Edw. and H. Family 13. Cystipiiyllid.y. —Solitary corals with rudimentary septa, and the calicle filled with vesicular endotheca. Genera —Oystiphyllum, Lonsdale (Silurian and Devonian). Goniophyllum, M. Edw. and H. (In this Silurian genus the calyx is provided with a movable operculum, consisting of w four paired triangular pieces, the bases of each being attached & to the sides of the calyx, and their apices meeting in the middle when the operculum is closed). Calceola, Lam. (In s this Devonian genus there is a single semicircular operculum furnished with a stout median septum and numerous feebly developed secondary septa. The calyx is triangular in section, pointed below, and the operculum is attached to it by hinge-like teeth.)