7. Anthozoa (incl. Hydrocorallia). A. Zoantharia. 1 9 



latter the products of fission never resemble bud- or larval polyps [see Bericht 

 f. 1902 Coel. p 22 Duerden( 6 )]. Apparently true discal gemmation may take 

 place in species which usually reproduce by columnar budding. 



Crossland gives general conclusions concerning the reefs of the whole 

 coast of East Africa. There are no reefs due to growth in situ. There have 

 been great coral reefs in geologically recent times consisting of a fringing reef 

 and a barrier reef in deep water, whose upheaval and consequent crystalli- 

 sation forms the rock of the whole coast and the outlying islands and reefs. 

 The islands of Pemba and Mnemba are formations independent of the above 

 system. All the reef forms typical of growing coral reefs are formed by phy- 

 sical agencies operating upon this dead rock aided by protection against ero- 

 sion afforded by organisms (principally algie) which grow especially where the 

 rock is exposed to the open sea. (1) Fringing reefs, east coasts of Zanzibar 

 and Pemba Islands; (2) Barriers (a) of the mainland, broken as in Pemba 

 Channel, regular as at Mombasa, (b) off the west coast of Pemba; (3) Atoll 

 formations in miniature near Zanzibar and in the mainland barrier. Recent 

 growth cannot form typical reefs in these regions under present conditions since 

 the surf is not strong enough to throw up sufficient coral fragments above the 

 level of low tide for the formation of reef conglomerate. 



Gardiner( 2 ) divides the coral banks of the Indian Ocean into 4 groups 

 Malagasy, Seychelles, Chagos and Maldive - - and gives a description of each. 

 The atoll of Minikoi, the most southern reef of the Laccadives is taken as an 

 example of this group and fully described; from a consideration of its blocks 

 of coral and limestone the author concludes that there must have been an ele- 

 vation of at least 24 feet and that the atoll existed as such when this ele- 

 vation occurred. The limestone of North Mahlos is similar to that found in 

 Minikoi, many of the corals present in it are entire, in the positions in which 

 they were originally growing and show that elevation has occurred. The open 

 banks of the Maldive Group show numerous small flat reefs arising from 25- 

 30 fms. A flat reef may become completely covered with land or a small 

 depression may appear in its centre due to the impossibility of organisms 

 growing there, to solution and to the outwash of sand and mud. This may 

 deepen, the reef meanwhile spreading and the bank is thus converted into a 

 small atoll (atollon or faro). Other faro are formed and neighbouring ones 

 meet and fuse forming a double rim to the bank which by this means is turned 

 into an atoll (vide Suvadiva Atoll). Finally the inner parts of the small atolls 

 are removed by solution and the action of currents so that their lagoons are 

 thrown into the large central one and thus the typical atoll is brought about. 

 Each of the larger reefs in the Mahlos Bank is a separate entity which has 

 grown up, probably in this way. It is only now that the bank is at all ap- 

 proaching the condition of the perfect atoll by the joining together of the small 

 atolls, as above described, to form long linear reefs, with the loss, perhaps, 

 of the whole inner part of the smaller atolls. In every atoll visited the en- 

 circling reef is closing itself up. Along with the gradual but sure increase in 

 the size of the lagoon the edge of the reef is growing seaAvards at least as 

 rapidly. From the outer edge of an encircling reef-flat there is generally to 

 seaward a gradual slope to 30-50 fathoms in 200-400 yards, succeeded by 

 the steep slope. The former is the growing area being almost covered with 

 living organisms, spreading and branching colonies of all kinds of corals which 

 are being bound together and fixed by lAihothamwion, Polytrcma and many 

 other kinds of encrusting organisms. The author concludes that in the case of 

 the Maldives and Laccadives an almost flat plateau was at one time formed 



d* 



