866 
Proceedings of the Royal Society 
and Orbicella Laperouseana (var).* The smaller piece of rock 
showed no coral structure to the eye, and was much honeycombed : 
it was incrusted with Nulliporce and Polyzoa. Another of the sub- 
merged reefs, covered by from 6 to 7 fathoms of water, was marked 
by streaks and patches of calcareous sand, and possessed apparently 
but little living coral. I obtained here branches of a Madrepora , 
which commonly occurred on the weather slopes of reefs in depths 
of from 8 to 12 fathoms ( vide p. 861). 
With reference to the character of the bottom in the deeper parts 
of Bougainville Strait, I should remark that in depths of from 20 
to 40 fathoms calcareous sand and gravel usually occur. These 
materials are largely composed of the tests of Orbitolites, and the 
joints of the calcareous alga Halimeda opuntia. f Small unattached 
corals of the genus Heteropsammia here flourish. J Fragments of 
Nulliporce were frequently brought up from all depths down to 86 
fathoms, where Lieut, keeper obtained a small nob apparently 
alive. The foraminifer, Polytrem,a rubra , was occasionally found 
attached to these fragments. 
In the sheltered waters of this strait there is no intermediate 
condition between the submerged reefs covered by 4 to 8 fathoms 
of water and those which are marked on the surface by a reef-flat 
with its accompanying islet, or by a sand-key. From the manner in 
which the channel was studded with submerged or sunken reefs, 
it was at first thought that the survey would be attended with some 
risk to the ship ; but Lieut. Oldham subsequently found that, on 
account of her small draught, H.M.S. “ Lark” could sail with safety 
over any of the reefs that were not marked at the surface by reef- 
flats or sand-keys. When I had examined these submerged reefs, I 
was led by their characters to conclude that they had reached the 
limit of their upward growth, and that they have since been extend- 
ing laterally, so as to form long and narrow platforms, some of which 
are more than 10 miles in length, and are in fact submerged barrier- 
* This was the only occasion on which I obtained this Orbicdla. Numerous 
other organisms were found in or upon this mass of rock, such as Ophiuroids, 
a Comatula, Olivce, Arcce, a small eel-like fish, &c. 
t This alga grows on reef-flats and in the wash of the surf. 
+ More than one species of Heteropsammia live in these depths of from 20 to 
40 fathoms, H. multilobata being common. The majority, if not all, were 
characterised by the presence of the Sipunculus ( Aspidosiphon ). 
