478 Mr Gardiner, The Coral Reefs of [Mar. 7, 



form structures, which, if upheaved, would give a rock precisely 

 similar to this. 



In the lagoons, it must be remarked, the shoals are formed 

 much more of corals and have everywhere precipitous sides. Nodu- 

 lose, mamillated and laminated species of Lithothamnion grow, 

 but, while they are often very abundant, they cannot have the 

 same binding effect as the incrusting forms. Algae of the genus 

 Halimeda, Mollusca and Sarcophytum abound, and the whole rock 

 seems a somewhat loose conglomerate of the remains of all these, 

 held together mainly by coral growth. 



SECTION II. 

 The Bathymetrical Limits of Coral Growth. 



Dana on this subject sums up the evidence to 1890 in these 

 words * : — " There is hence little room to doubt that 25 fathoms or 

 150 feet, may be received as the limit in depth of flourishing 

 banks of reef corals." Although I cannot admit some of the 

 arguments that he urges in favour of this view, there is really no 

 evidence which proves that "flourishing banks of reef corals" 

 grow at a greater depth ; indeed it would be equally correct, if 

 Dana placed the depth at 15 fathoms, which is about the limit of 

 the visible zone. 



Yet I think that, considering the few dredgings that have 

 been taken outside coral reefs and in spite of the few species 

 of incrusting and massive corals, which have been brought up, 

 there is now strong presumptive evidence that corals live down 

 to depths of 40 — 50 fathoms in great luxuriance. Dr Bassett 

 Smith's dredgings on the Macclesfield Bank showed that 18 genera 

 of corals with 40 species were living at depths from 20 — 44 

 fathoms 2 . "The following 10 genera were found at a greater 

 depth than 30 fathoms : — Stylophora, Astraea, Pavonia, Gycloseris, 

 Leptoseris, Stephanaria, Psammocora, Montipora, Alveopora and 

 Rhodaroea, besides seven small scarcely reef-building genera." 

 The total number of species collected was 142, so that nearly one- 

 third were represented in depths over 20 fathoms. 



The character of the reef off Rotuma is certainly such as to 

 indicate a growing coral bed to depths up to 40 fathoms, and the 

 soundings and dredgings off Funafuti give for this atoll a similar 

 depth. The dredge and the swabs when placed down in depths 

 of 20 — 40 fathoms were constantly caught up, and their whole 

 behaviour indicated such a very rough and uneven bottom, that 



1 hoc. cit. p. 118. 2 Nature, Vol. xl. p. 223. 





