In order to give an idea of the general form of these atolls, although they are rarely so regular, the reader is referred to PL. III., which represents one of these islands of the Pomotouan Archipelago, in the Indian Ocean. It represents the island of Clermont-Tonnerre, figured by Captain Wilkes in the American Exploring Expedition. The exterior girdle of rocks here surrounds a basin nearly circular. Such is the general form--the typical form, so to speak--of the coral isles, of which this is a fair representation. The zoophytes which form these mineral accumulations belong to diverse groups, and nowhere have the results of observations made upon these atolls been more minutely described than in Mr. Darwin's remarks on the grand Cocos Island situated to the south of Sumatra, in the Indian Ocean. No writer, it seems to us, has reasoned on these atolls more comprehensively than the author of the "Origin of Species." "The earlier voyagers," he says, "fancied that the coral-building animals instinctively built up their great corals to afford themselves protection in the inner parts; but so far is this from the truth, that those massive kinds, to whose growth on the exposed outer shores the very existence of the reef depends, cannot live within the lagoon, where other delicately-branching kinds flourish. Moreover, in this view, many species of distinct genera and families are supposed to combine for one end; and of such a combination not a single instance can be found in the whole of nature. The theory that has been most generally received is, that atolls are based on submarine craters, but when the form and size of some of them are considered, this idea loses its plausible character. Thus, the Suadiva atoll is forty-four geographical miles in diameter in one line by thirty-four in another; Rimsky is fifty-four by twenty miles across; Bow atoll is thirty miles long, and, on an average, six miles broad. This theory, moreover, is totally inapplicable to the Northern Maldivian atolls in the Indian Ocean, one of which is eighty-eight miles in length, and between ten and twenty in breadth."