7. Anthozoa (incl. Hydrocorallia). A. Zoantharia. 27 



backward and forward flow of water over reefs prevents planulse from settling 

 on smooth, open, horizontal surfaces, but they will settle on, the lower surface 

 of tiles placed on the reefs. But though such a position is favourable for the 

 initial growth of corals, it is not the most favourable for their ultimate survival, 

 on account of the are algse, sponges and polyzoa which have a more rapid 

 growth, and smother more slowly growing organisms. The relatively small 

 number of planulye which attach themselves in crevices have the best opportu- 

 nity for growth and ultimate survival. To offer a proper foundation for a 

 coral reef, the sea-bottom must, besides being hard and clean and having a 

 free circulation of water over it, be also rough. A naturally attached yearling 

 Eusmilia knorri, derived from a single planula, was 22 x 18 mm. in diameter, 

 and 18mm. in height. Corals, e.g., Agaricia, accustomed to diminished light, 

 are killed or injured by being placed in strong light. A specimen of the thin, 

 sub-circular fragilis, cemented to a tile in June 1910 and returned to the sea, 

 had, by June 1911, assumed the crassa growth- form. In very quiet water, 

 this species of Ag. is thin, orbicular or reniform, with a slight basal attach- 

 ment at its centre, while on the reefs it is more strongly attached, and has a 

 more massive growth-form. But in quiet waters, the massive growth-form may 

 be produced by giving the normally thin form a wide base of attachment, i. e., 

 there is a reaction to contact. On the reefs, where the water is strongly agitated, 

 the peripheral polyps probably cling to the basal support, thus causing the basal 

 attachment to cover a larger area than it does in quieter waters; upward growth 

 from this wide base would produce the massive form. There are further 

 records of corals growing naturally, and a series of photographs illustrating one 

 year's growth of examples of -P 7 ., P., A. and Mceandra. 



Vaughan( 2 ) discusses the physical conditions under which palaeozoic coral 

 reefs were formed and concludes, that the conditions were similar to those under 

 which recent reef-corals live, namely (1) a maximum depth of 25 fathoms, 

 and strong light; (2) an annual minimum temperature not lower than between 

 15 and 17, and probably above 17; (3) agitated and circulating water; 

 (4) a bottom which was relatively free from deposits of silt; (5) the salts in 

 the ocean were probably the same as those in the ocean of the present day; 

 (6) the specific gravity of the oceanic waters in palaeozoic and in present times 

 was probably about the same; certainly the water furnished the large quantities 

 of calcium salts required by the reef-organisms. The author briefly sketches 

 the stratigraphical and geographical distribution of the chief reef-building corals. 



Fryer points out that, unlike the Maldive or Seychelles series of islands, the 

 Aldabra series has no shallow bank from which its various members arise. 

 There are, however, indications of a deep bank, about 1000 fms. from the 

 surface of the sea; all the islands are therefore separated by depths of at least 

 1000 fms. Aldabra is an atoll with an unusually perfect land-rim, divided 

 by narrow channels into 4 parts. There are numerous islands in the shallow 

 lagoon, and on the shores of the latter a mangrove-swamp. The average height 

 of the land above high water-mark is 12 ft. The outer portion of the land- 

 rim is composed largely of reef-corals, which are generally still in the same 

 positions as when alive; this rock has therefore been formed by elevation 

 of a reef, which, as the facies of the corals shows, had been formed in quite 

 shallow water. On the inner side of this rock (but mainly at the east end of 

 the atoll) is rock formed to a large extent of the debris of corals, and over the 

 whole atoll there is metamorphosed coral-limestone containing inclusions of 

 calcium phosphate. The lagoon-shores are coated with mud, charged with carbon 

 dioxide and various organic acids (derived from decaying organic matter), which 



