THE BRAIN CORALS. 59 
As there is not extant a published account of these interesting islands, which are so 
intimately associated with what we have to say about the marine objects of the semi-tropical 
waters of North America, we feel sure that it will be acceptable to the reader to have a some- 
what detailed account of them. 
The Dry Tortugas, before the late conflict of 1861-5, was little known to the average 
reader. The establishment of a military prison there soon made the name a terror to evil- 
doers, and a synonym for the dreadful. During the two years preceding the ‘‘ conflict ”’ 
it had been our fortune to reside at the Tortugas as United States surgeon. Fort Jeffer- 
son was then in progress of construction. It is an enormous work, involving many millions 
of money. During these two years the quiet life and delightful association with other officers 
of the post, and their families, interested in the same pursuits, rendered it an opportunity of 
exceptional excellence for the study of marine zoology. 
The visitor to this region in years when the post was garrisoned would take the following 
course: Usually a stop, coming from the north, was made at Key West, the only important 
inhabited island then on the reef. From there a sail, usually by night, of sixty miles, brought 
one off Marquesas Keys and Rebecca Shoals. Daylight reveals in the western horizon a long 
row of castellated structures, impressing oneas fairy castles, now illumined by the rays of the 
rising sun. The vessel now abruptly changes her course, to enter the peculiar winding 
channel that is so characteristic of the coral reef—five miies from the fortress. Anon there 
shoots forth a small cloud from the top of the work, and simultaneously rises the garrison 
ensign, followed at an interval by the booming sound of the sunrise gun. 
In this delightful climate, even during the winter months, this scene is as enjoyable as it 
is novel. On all sides is the vast ocean. Not a sign else, save the four green-capped islets, 
slender white strips on the blue sea, with low green bushes on their surface. These now begin 
to be distinguished. Seven of those small islands, of sizes varying from a quarter of a mile to 
two miles in length, form a sort of irregular ring around a deep harbor. The intervening 
space is occupied by the solid reef that has been built up from the sea-bottom, and lies just 
under the surface, many miles in extent, the entire group being about circular and some seven 
miles in diameter. The water on this area varies in depth from one foot to twenty, and it is 
the abode of great numbers of the shoal-water corals, corallines, and algze. In the centre, or 
nearly so, of the harbor, an islet of sand, formed like all the others on the solid coral basis of 
the reef, and about thirteen acres in extent. On this island, entirely covering it, is built Fort 
Jefferson, the largest structure of the kind in the United States. 
Though these little islands look to us like mere sand-spits that any stout gale might 
demolish, they are grounded in the most endurable of material. The solid area of extended 
reef around them, just beneath the surface, is as firm as rock. Just at the edge of these 
islands, on the windward side, the waves break with great violence—the vast ocean depths 
are behind. The still waters within offer the safest anchorage, reached through the narrow, 
winding channels. The nature of coral reefs the world over is to grow in such shape as to 
inclose lagoons with more or less depth of water, which is usually sufficient to float the largest 
vessels. Hence the great value attached to coral islands in the great Indian Ocean, where 
_passing vessels seek temporary shelter from storms. 
The harbor within these islands is valuable for the navy in time of war, as otherwise the 
presence of a great fortification here is useless. 
The important elements in the building up of these coral reefs are the Astrean Corals. 
These are not circumscribed in growth like many others, but are seemingly indefinite in 
boundary. Immense ledges are seen cropping out of the mud in shallow water. 
The Brain Corals, so called from their resemblance to the brain, Meandrinas, from the 
meandering nature of their cells, exhibit a number of beautiful shapes, ranging from the most 
regular hemispheres to masses of indefinite shape and size. These, with the star corals, the 
astreas, as we have seen, form important elements in the building up of reefs. 
In the coral regions of the West Indies and the Florida peninsula the islands are called 
cays, in English keys, a corruption from cayo, Spanish for an islet. The principal cay of 
Florida, or the only considerable one inhabited, was early called Cayo hueso. Bone Cay, or 
