864 
Proceedings of the Royal Society 
particularly hardy in this respect. At “low water springs” the 
extensive beds of a species of Madrepora , that thrives on reef-flats, are 
uncovered to the extent of 5 or 6 inches for an hour and a half ; and 
other species, such as M. hyacinthus (D.), will hear exposure equally 
well. Of all the corals in these islands, however, those belonging 
to the genus Goeloria appear to be best able to withstand prolonged 
exposure. I have seen living blocks of these massive corals bare to 
the extent of from 12 to 18 inches at the lowest tides. A lump of 
Goeloria dcedalea (E. & S.), that I observed on one reef, had its 
upper surface exposed for almost two hours ; and I have frequently 
noticed the living margin of another species of the same genus to 
be uncovered to the extent of several inches for about an hour. 
On one occasion, five hours after I had taken two lumps of this 
last species out of the water, I placed them with some other corals in 
a closed perforated tin that w T as kept in the sea, a foot or two below 
the surface, for a couple of days. On opening the tin, I found all 
the specimens of coral decomposed, with the exception of one of 
the lumps of Goeloria , which was quite fresh and apparently still 
alive. 
The soft beds of Alcyonarians are sometimes completely exposed 
for 5 or 6 inches at the lowest tides. The Xenice , however, I rarely, 
if ever, saw bared. Many other organisms living on the reefs are 
able to withstand inter-tidal exposure. Thus, in Oima Atoll I saw 
tufts of Sertularians and the common blue star-fish (Ophidiaster ? ) 
uncovered at “ low water springs.” At the same time two of the 
large Tridacnce (T. gig as), that lay on their hinge-borders upon the 
sand, were partially exposed. One of them was half out of the 
water, and must have been uncovered for at least an hour. 
Goral Reefs and Shoals. — The earliest condition of the coral reefs 
in this group is to be found in that of the numerous detached sub- 
merged reefs or shoals lying below the limit of the constructive 
power* of the breakers, which in the sheltered waters between the 
larger islands rise up to within 4 or 5 fathoms from the surface, and 
in the more open waters, where they are exposed to the trade-swell, 
are covered by a depth of from 7 to 10 fathoms. 
* By this expression I mean that the reefs had not reached that level at 
which the breakers would be able to tear off and heap up fragments of the 
corals. 
