of Edinburgh , Session 1885 - 86 . 
859 
however, as at low- water springs, some large block is torn off by the 
rollers and cast upon the reef: it is at these unusually low con- 
ditions of the tide, especially when they follow upon a succession of 
gales, that the greatest destruction is effected on the weather slope. 
There is, for instance, a very stout species of Pocillopora that seems 
to flourish just beyond the ordinary position of the first line of 
breakers. When, however, at low-water springs the seas are 
unusually heavy, fragments of its large branches are broken off and 
thrown up on the flat. The two algae, Halimeda opuntia and a 
species of Caiderpa , are often attached to the rocks in the wash of 
the surf, and Nulliporce partially encrust the surface ; but the last 
mentioned are of no thickness, and never form those raised margins 
described by Mr Darwin in the case of Keeling Atoll and of other 
reefs. 
The foregoing remarks refer to such reefs as receive the 
brunt of the trade-swell. There are other reefs, however, such 
as the barrier-reef of Choiseul Bay, which are placed in more 
protected situations, where they are not exposed to such heavy 
rollers. Here the corals living in the wash of the breakers are 
more numerous and in greater variety. In addition to the corals 
above mentioned, there are to be found here a gamboge-coloured 
Porites ( P . parvistella , Quelch), growing in flattened nobs, together 
with small masses of that singular-looking coral Hydnophora 
miarocona (L.). Madreporce , such as usually prefer the quieter waters 
of the lagoon, here attempt to flourish ; but they are to be frequently 
observed broken off at their base of attachment. There may be 
also noticed, growing in the wash of the rollers, the vertical plates 
of the hydrozoan corals, Pleliopora ceerulea * and Millepora platy- 
phylla, Ehr.)f 
On the lee sides of small coral islands, where they are protected 
to a great extent from the swell, the massive and branching corals 
coexist in great profusion. Here, on account of the absence of the 
heavy rollers, they do not form a continuous reef-flat, but are 
* Mr S. Ridley informs me that there are two apparently distinct species 
in my collection, one being probably new. 
t The Stylasteridse, which Professor Mosely has placed with the other 
hydrozoan corals in the sub-order Hydrocorallinse, did not frequently come 
under my notice among the reefs of these islands, and then only as small 
specimens. 
