176 J. STANLEY GARDINER. 
The question as to the depth at which reef-corals occur is an important one, and one 
to which we devoted very particular attention. With open boats and a sailing vessel it was 
practically impossible to dredge the steep outer slopes of the reef so as to get any sample 
of the bottom. There was too no other ground in the Maldives with a depth of between 
50 and 100 fathoms, so that necessarily our work was confined to depths below the lesser 
limit (Ze. 0 to 50 fathoms). The dredgings between 30 and 50 fathoms outside reefs were 
very few, but there were a large number in the passages of the atolls and a still larger 
number in the lagoons. Of the latter only those close to shoals (surface-reefs) would be 
expected to yield any evidence, since it is only in such positions that a hard rocky bottom, 
suitable for coral growth, is found. Heliopora! was obtained off Addu, growing apparently 
luxuriously at about 40 fathoms, but of genera of true corals at all important on the 
surface reefs in any region Madrepora, Seriatopora and Pocillopora alone were represented 
below 30 fathoms. It might possibly have been expected that only branching corals would 
have been obtained, and that no evidence would hence be forthcoming as to the real depth 
at which most reef-corals live. Yet in shallower depths colonies of massive species were not 
infrequently procured, and the characters of the growths of the above-mentioned genera 
afford the strongest possible deductive proof that reef-corals do not live below about 50 
fathoms nor flourish 20 fathoms higher’. It was surely remarkable too that such branching 
corals as Montipora, Porites, Pavonia, Psammocora, Mussa, Euphyllia, etc. besides the 
hydrocoralline Millepora, all common enough on reefs, should not have been represented at 
all below 30 fathoms. The Madrepora secured belonged to finely-branching facies with hght 
coralla, and only a single small piece of a delicate Seriatopora was procured. The Pocillopora 
afforded less definite evidence, several pieces of massive facies being brought up, though most 
colonies were finely branching. Yet compared with the large pieces of coarse Pocillopora, 
apparently of the same species brought from 20 fathoms, its growth is very evidently far 
from luxuriant at this depth. Dr Bassett-Smith’s dredgings on the quite open Tizard and 
Macclesfield banks, in which he had the use of steam, support the same view, fragments 
of Favia and Montipora alone being caught up between 41 and 50 fathoms and only six 
so-called species from 31 to 40 fathoms®. At Funafuti eight dredgings 40 to 140 fathoms 
on the steep slope of the reef yielded no reef-corals, and with “swabs” 30 to 45 fathoms 
I only secured about 20 fragments of Madrepora, Pocillopora, Stylophora and Porites, with 
pieces of Millepora and Heliopora*. The characters of the specimens were the same as 
those subsequently procured in the Maldives, and they are scarcely such as characterise the 
corals that maimly build up the reefs as we find them in the present day. 
It would seem to me then that about 30 fathoms is the extreme limit in depth of 
the growth of the effective reef-building corals. Our dredgings also brought out the fact 
that the luxuriance of growth of corals progressively increases as the water becomes shallower 
to within a few (3—6) fathoms of the surface. The formation of corallum in corals is, I 
consider, dependent on the excretion of ammonium carbonate, and is directly proportional 
to the amount of the metabolism. The latter, so far as the reef-forms are concerned, is 
1 Vide note on p. 168. forms too have finer and more delicate stems, consistent 
2 This question will be better brought out and dealt with with the limits of variation found in any species. The 
in the “Report on the Madreporaria,” which I am now polyps also have a tendency to be further distant from 
preparing. I may here mention that, with any genus or one another. 
species of such corals, the skeleton decreases in density 3 Ann. Mag. Nat. Hist., Nov. 1890. 
(i.e. specific gravity) with increase of depth. Branching 4 Loc. cit. p. 479. 
