258 Mr. John 3Iurray [Marcli 16, 



of lime shell or skeleton or their broken-down parts. " Coral " is 

 used by the naturalist in a much more restricted sense ; he limits the 

 term to animals classed as Madrepores, Hydrocorallines, and Alcyo- 

 narians. The animals belonging to the first two of these orders 

 comprise those included under the vague term of reef corals. Be- 

 sides these, however, very many other classes of animals contribute 

 to the building up of coral reefs and islands — such are Foraminifera, 

 Sponges, Polyzoa, Annelids, Echinoderms, and Calcareous Algae. The 

 relative proportions of these different organisms in a reef vary with 

 the region, with the depth, and with the temperature, but members 

 of what are known under the term of reef corals appear always to 

 predominate. 



The animals of the true reef-building species resemble the common 

 sea anemones in structure and size ; the individual polyps may vary 

 from the eighth of an inch in diameter to over a foot. Some of the 

 structures built by colonies may exceed 20 feet in diameter. 



There may be great variety in the appearance of submerged reefs 

 as they rise from banks of a different nature, form, and extent, 

 as, indeed, was pointed out long ago by Chamisso. There may be 

 differences due also to the kinds and abundance of deep-sea animals 

 living on such banks, as well as differences due to currents, tempera- 

 ture, and other meteorological conditions. 



From the very first the plantations situated on the outer edge will 

 have the advantage, from the more abundant supply of food and the 

 absence of sand in the water, which last more or less injuriously 

 affects those placed towards the interior. Chamisso attributed the 

 existence of the lagoon to the more vigorous growth of the peri- 

 pherally situated corals of a reef, as compared with those placed 

 towards the middle, and in this he was to a large extent right, but 

 the symmetrical form of the completed atoll is chiefly due to the 

 solution of the dead carbonate of lime structures. The Great Chagos 

 Bank illustrates the irregular way in which such a large bank of 

 coral plantations approaches the surface. When these, however, 

 reach the surface, they assume slowly a more regular outline, those 

 on the outer edge coalesce, and ultimately form a complete ring of 

 coral reef, and the lagoon becomes gradually cleared of its coral 

 patches or islands, for as the atoll becomes more perfect, the con- 

 ditions of life within the lagoon become less and less favourable, and 

 a larger quantity of dead coral is removed in solution. 



The coral atoll varies greatly in size and form : it is usually more 

 or less circular, and may be one or over fifty miles in diameter. 

 The breakers spend their fury on the outer edge, and produce what 

 is known as the broad shore platform ; but within, trees descend 

 to the very shore of the lagoon, where there is quiet water, and a ship 

 may often enter on the lee side of the atoll and find safe anchorage. 



In this connection it is important to bear in mind the relation 

 which exists between the periphery and the superficial area of the 

 lagoon in atolls of different sizes. If the coral plantations which rise 



