GROWTH-CONDITIONS OF REEF-CORALS. 71 



This highly characteristic structure of the larger reef-m asses, typified by a relatively large 

 dead and weathered central core, associated with a comparatively small and narrow peripheral 

 zone of vitality, is anticipated in an instructive manner by the developmental history or 

 ontogeny of an individual coral-stock depicted in several of the accompanying photographic 

 reef-views. 



The massive growths of Goniastra;a in the lower figure of Plate IV. illustrate this pheno- 

 menon most conspicuously. The foreground coral-stock, in this instance, perfectly typifies those 

 varieties of coral-reefs in which the central area is almost entirely dead, and weathered by exposure 

 to sun and air. The lower figure of Plate V., and the corresponding one of Plate VI., reproduce 

 in miniature, in like manner, those numerous reefs in which, while the peripheries represent the 

 essential growing zones, the dead and weathered horizontal central areas support scattered patches 

 of coral species distinct from that of their own basal mass, which in these two instances is a 

 species of Porites. Those reefs in which growing coral is developed almost uninterruptedly over 

 extensive areas, as illustrated more particularly by such plates as Nos. XII. and XVII., are very 

 rarely laid bare by the receding waters, and then only during abnormally low spring or (so-called) 

 " king-tides." 



The highest elevation at which corals are found growing in the Barrier district is 

 that of about ordinary low-water mark. Thence downwards, to a depth of some twenty or 

 thirty fathoms, represents the generally recognised bathymetrical range of reef-coral growth. 

 Their most luxuriant development, however, is limited by a depth of about fifteen fathoms from 

 low-water — an area that corresponds essentially with what is known as the Oar-weed or 

 Laminarian zone of European and other temperate seas. Quite recently, a limited development 

 of reef-corals, as represented by the genus Madrepora, has been found at a depth of from 

 forty to forty-five fathoms — the area of this abnormal growth was the Macclesfield and 

 Tizard banks of the China seas. The reef-corals obtained from this depth exhibit a distinct 

 aspect as compared with those of the shallower-water zone, being, as a rule, of much 

 slighter build. 



There are not a few Madreporaria, or Stony-corals, whose growth-zone is coinci- 

 dent only with the cold deep water, and abyssal depths, of from one hundred to between 

 two and three thousand fathoms. The species occurring at the greatest depths are, for the 

 most part, simple or solitary species of extreme tenuity, which could exert no material influence 

 on reef-construction. However, within the more moderate soundings of from one hundred 

 to five or six hundred fathoms, there exist, even in European seas, conspicuous species 

 whose deep-water habitat alone excludes them from participating substantially in the work 

 of reef-building. Dendrophyllia, Lophohelia, Amphihelia, and other allied genera form massive 

 branching coralla, or thick bushy growths, extending over large submarine areas which, if 

 raised above the surface, would present as luxuriant and picturesque a reef-scape as is shown by 



