The star-corals. The corals of this genus are hemispherical masses covered with small star-shaped pits, or corallites. Although the diameter of an astræa-dome may be twelve feet or more, it has only one half or three quarters of an inch of living coral on its surface, the rest being solid matter left behind as the polyps rose in growth. The colony increases by self-division. The septa meet in the center of the corallite, making star-like pits; the surface is comparatively smooth. (Plate XLIV.) Genera ^Meandrina^, ^Diploria^ Corallum massive, hemispherical in shape, with furrows running in irregular lines over the whole surface. The peculiar serpentine form of the corallite is produced by the animal growing in one direction, fission being incomplete, and new mouths being successively opened until a line of them extends along a common stomach. ^D. cerebriformis.^ This species is commonly known as brain-coral or brain-stone. The hemispherical shape, together with the peculiar serpentine corallites, makes its resemblance to the human brain very noticeable and the name unusually appropriate. This species, when living, is bright yellow. (Plate XLV.) [Illustration: PLATE XLIV. Millepora alcicornis. Lucernaria auricula. Cassiopeia frondosa. Astræa argus. Oculina.] [Illustration: PLATE XLV. Porites furcata and P. astræaoides. Diplora cerebriformis. Madrepora cervicornis. Madrepora palmata. Mycedium fragile.] GENUS ^Madrepora^ Some species of these corals appear like branches, while others have flat, low surfaces or fronds; all of them are covered with innumerable small cylindrical cups. Each cup is the home of a polyp, which secretes calcareous matter around its column, and septa between its radial partitions. The corallum, when examined with a glass, looks like meshwork. Through the fine openings of the porous surface, the polyps of the colony are connected by interlacing tubes of animal substance (cœnosarc). The polyps of the madrepore colonies are small in comparison with the connecting structure, which becomes very massive; but, although large, the corallum is fragile, on account of its perforated character and the exceeding delicacy of its parts. In species which form incrustations, new polyps arise from tissue which spreads beyond the corallites. In some species certain polyps and corallites increase in length, growing longitudinally, and continually form buds around the base. This manner of growth leads to the formation of branches, while the former manner gives lateral extension. Both of these forms of budding may take place at the same time in the same colony. The madrepores are among the most common of the reef-builders. They form reefs which extend for miles around the Florida Keys.