Madrepores (which must not be mistaken for corals) have a tissue lined with a calcareous crust, and the modifications of its structure have induced M. Milne Edwards, my worthy master, to class them into five sections. The animalcule that the marine polypus secretes live by millions at the bottom of their cells. Their calcareous deposits become rocks, reefs, and large and small islands. Here they form a ring, surrounding a little inland lake, that communicates with the sea by means of gaps. There they make barriers of reefs like those on the coasts of New Caledonia and the various Pomoton islands. In other places, like those at Reunion and at Maurice, they raise fringed reefs, high, straight walls, near which the depth of the ocean is considerable. Some cable-lengths off the shores of the Island of Clermont I admired the gigantic work accomplished by these microscopical workers. These walls are specially the work of those madrepores known as milleporas, porites, madrepores, and astraeas. These polypi are found particularly in the rough beds of the sea, near the surface; and consequently it is from the upper part that they begin their operations, in which they bury themselves by degrees with the debris of the secretions that support them.