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Show 736 MR. J. STANLEY GARDINER ON [June 6, 1*1-1-9 cm. in diameter. The costae of the first two cycles of septa are distinct down the whole of the corallum, but those of the third cycle extend only for about 3 m m . and are very small. The septa vary up to 2 m m . in exsertness and are entire, rounded at the top, sloping down rapidly to the axial fossa, which has no columella, being closed in below by trabecule from the septa. In the living colony there are often two or three mouths, where there is no trace of more than one system in the corallum beneath. The polyps, when fully expanded, rise for about 3 cm. above the skeleton^ but contract to less than 1 cm. in spirit; their tentacles are about 2 cm. long and not retrusible. The species is exceedingly difficult to preserve on account of the enormous quantity of mucus secreted, the least touch with the hand under water causing the complete obliteration of the stomodseum and peristome. The colour of the polyps varies from green-yellow to olive-brown, always markedly greener towards the stomodaea. Rotuma; in pools of outer reef, very local in distribution, often forming large masses, 3-4 feet across. I have been unable to determine the method by which the calices are divided, but in elongated corallites two of the larger septa sometimes fuse across the fossa and form a division. The thecal walls of the two sides then grow horizontally over the edges of these and fuse, flattening out to a breadth of about 2 m m . The young corallites next broaden out in a plane at right angles to the elongation of their parent corallite, themselves later to undergo a similar division. Genus M U S S A. Mussa (pars), Oken, Lehrb. der Naturg. i. p. 73 (1815). Mussa, Dana, Zooph. p. 173 (1848). Mussa, Milne-Edwards & Haime, Cor. ii. p. 328 (1857). Although I have examined a very large number of specimens of this genus both in the British and Cambridge Museums, I have completely failed to find any characters of good general specific value. Tet the specimens seemed to fall naturally into from eight to ten groups, none of which, however, were sharply marked off from their neighbours. The characters of the several septa, on which Dana very largely relied, do not seem to m e to be of any great importance, and not uncommonly in the same colony septa corresponding to 4 or 5 of Dana's species can be found. The depth and shape of the corallites, too, depend very largely on the position of growth, i. e. whether the colony has plenty of room or is crowded with other organisms. Mainly depending on these characters Milne-Edwards and Haime described no less than thirteen species, most of which will indubitably be found to be synonyms. Although I depend partially on the general form of growth, yet the more important characters seem to m e to be-the extent and form of the epitheca, presence or absence and spinulation of costse, and characters of the columella if present. The arrangement |