of Edinburgh, Session 1885 — 86 . 
897 
Cidaris and Echinometra dig holes in the coral rock of the Florida 
reefs. In the Solomon Island reefs, Echinometrce occasionally 
occupy round deep holes of their own workmanship, hut cavities of 
this shape are generally formed by other species of echinoids. 
Submarine Deposits at present forming outside Coral Beefs . — 
The calcareous sand and gravel that strew the outer slopes "below 
the zone of living corals in depths between 20 and 100 fathoms and 
beyond, are largely composed of reef-debris, the tests of two species 
of Orbitolites ( Orbitolites complanata and heterostegina ), the joints 
of the calcareous alga, Halimeda opuntia , and of Nidliporoe. I have 
found the two species of the Foraminifera just mentioned living at 
all depths between a couple of feet and 75 fathoms ; and frequently 
when I have been using a lead having a broad cup 3J inches across, 
ten or twelve of their tests have been brought up together on the 
arming. Their bleached tests, for when alive or recently dead these 
Foraminifera are of a sea-green colour, are equally numerous in the 
chalk-like mud exposed inside the reefs by the retreating tide, and 
together with the joints of the Halimeda they may be said to largely 
compose many beaches.* I have only found this calcareous alga 
living on reef-flats and in the w r ash of the breakers on the outer 
borders of reefs and among the troubled waters caused by tide-rips 
on coral shoals, which are covered by less than 10 fathoms of water. 
In my soundings on the outer slopes of reefs I only brought up the 
dead joints. It may be truly said that next to coral debris the tests 
of Orbitolites , fragments of Nulliporce , and the joints of Halimeda 
opuntia are the most important rock-forming materials in depths of 
from 20 to 100 fathoms on the outer slopes of reefs in these seas.f 
* The Foraminifer, Tinoporus bacidatus, is also commonly found in the 
sand of the Solomon Island beaches. 
t Similar deposits of Orbitolites are forming off the Australian shore between 
the inner reefs of the great barrier-reef. Mr Beete J ukes found that the dredge, 
from depths of 15 or 20 fathoms, was sometimes filled with Orbitolites : these 
organisms seemed in some places to make up the whole sand of a beach 
of a coral island (Student's Manual of Geology, 1862, p. 131). The 
important part taken by Halimedoe and Nulliporce in the composition of 
deposits of coral reefs attracted the attention of Mr Darwin. We are informed 
by him that joints of a Halimeda and small fragments of Nulliporce, all dead, 
thickly strew the bottom in depths greater than 90 fathoms, off Keeling Atoll ; 
and also that Capt. Allen, R.hT., in his survey of the West Indies, found that in 
depths between 10 and 200 fathoms the armings very generally came up 
covered with the dead joints of a Halimeda (Coral Reefs, 1842, p. 86). 
VOL. XIII. 3 N 
