DESCRIPTION OF THE EXPEDITION, 133 
them regular reef-builders. The Hydrocoralline Millepora and the Aleyonarian Helio- 
pora, as important reef-builders as any, were present both in this and in a subsequent haul 
in 29 fathoms. The former is particularly interesting on account of the extraordinary 
different facies which it assumes, incrusting, branching, massive, spreading, close-set 
leaves, &c. Any facies may be found at any depth and all belong, as Prof. S. J. Hickson 
has shown, to a single species. The second, the weli-known blue coral, decreases in depth 
of coloration with the increase in depth from which it is obtained, the colour being 
absolutely in the corallum and not in the living tissues. Its main pigment, known 
as helioporin, is peculiar to the genus and is accompanied by a pigment of a chloro- 
phylloid nature. ‘The upper surface of its leaves is of deeper blue than the lower, 
and this indicates that the pigment may exercise some function in decomposing the 
Sorting dredging C 16 on board H.M.S. Sealark, Saya de Malha. 
carbonic acid in the water, presumably for tke benefit of its living tissues *. Attached to 
the dead coral were some Jow purple Polytrema (Foraminifera) and some bluntly- 
branching, brick-red Lrythropodiwm which Prof. J. Arthur Thomson has informed me 
is growing on a madreporarian axis. 
After the last dredging we steamed slowly westwards during the night (Sept. 6-7), so 
as to dredge the following day over the edge of the bank and to explore the passage 
towards the northern bank. After some dredgings within the rim, we took one 
absolutely upon it in 29 fathoms, getting 14 species of corals and much the same 
forms as in our dredging at 26 fathoms on the previous day. We then passed off the 
bank, finding a regular coral-reef slope, and got a successful trawling commencing at 
* Vide C. A. MacMunn, ‘ Fauna and Geogr. Maldives and Laccadives,’ vol. i. p. 188. 
