The fundamental simplicity of structure, which is common to every coral, does not preclude the evolution of an amazing variety of forms. In the course of time as many species have been evolved as there are possible combinations of the conditions, animate and inanimate, which affect coral growth and survival. In form these range from huge and solid stones, weighing many tons, to tiny delicate things like petrified lace or ferns, some of substance nearly as hard as shells, others so spongy as to be easily cut into by a knife. I have been enabled to give two plates illustrating a few out of this amazing variety. Both massive, hemispherical or dome-shaped, and more delicate branched species are shewn, but lobed growths, such as that shewn at the bottom of the first group, intergrade the two divisions. The latter specimen is of particular interest, being a species of the genus _Porites_, already referred to as forming great cylinders of solid stone. This small specimen was taken from shallow water, near lowest tide level, so that the polyp cups, which are too small to be visible in the photograph, were intact only on its sides. Above they were killed by the air and sun, and the stone they had formed, being exposed to the action of the sea, has been dissolved away slightly, leaving a narrow rim round the edge, where the part that was protected by living flesh shews the height the colony originally attained. In the middle of the plate are small dome-shaped colonies, of species which, though rarely growing to the size of the _Porites_ cylinders, may form very considerable boulders. Notice the different shapes of the polyp cups, with their radiating plates, and the varying beauty they impart to the surface of the stone, a beauty which is enhanced by examination under a lens. _Plate XXVIII_ [Illustration: Fig. 60. Stony corals All are forms of genus _Madrepora_ except the lowest, which is a species of _Symphyllia_] Most of the branched kinds belong to one great genus _Madrepora_, one of the most conspicuous of all the forms seen in a living reef, and which contains a very great number of species. In spite of the wide variety of outer shape, the structure of the coral polyps is almost identical throughout this genus. The colonies may be quite small and are generally of moderate size, but one tremendous growth has been recorded, which covered an acre of the sea bottom and sent its branches of stone to the height of 50 feet. In this case a single coral equalled in size a plantation of large trees, but what is usually seen is a network of branches springing from one thick stem and spreading horizontally, and covering a fan-shaped or circular area of a square yard at most.