they now establish themselves on the top of the Coral wall and continue its growth for a certain time.  These are the Mandrinas, or the so-called Brain-Corals, and the Porites.  The Mandrinas differ from the Astraeans by their less compact and definite pits.  In the Astraeans the place occupied by the animal in the community is marked by a little star-shaped spot, in the centre of which all the partition-walls meet.  But in the Mandrinas, although all the partitions converge toward the central opening, as in the Astraeans, these central openings elongate, run into each other, and form waving furrows all over the surface, instead of the small round pits so characteristic of the Astraeans.  The Porites resemble the Astraeans, but the pits are smaller, with fewer partitions and fewer tentacles, and their whole substance is more porous.

But these also have their bounds within the sea:  they in their turn reach the limit beyond which they are forbidden by the laws of their nature to pass, and there they also pause.  But the Coral wall continues its steady progress; for here the lighter kinds set in,—­the Madrepores, the Millepores, and a great variety of Sea-Fans and Corallines, and the reef is crowned at last with a many-colored shrubbery of low feathery growth.  These are all branching in form, and many of them are simple calciferous plants, though most of them are true animals, resembling, however, delicate Algae more than any marine animals; but, on examination of the latter, one finds them to be covered with myriads of minute dots, each representing one of the little beings out of which the whole is built.

I would add here one word on the true nature of the Millepores, long misunderstood by naturalists, because it throws light not only on some interesting facts respecting Coral Reefs, especially the ancient ones, but also because it tells us something of the early inhabitants of the globe, and shows us that a class of Radiates supposed to be missing in that primitive creation had its representatives then as now.  In the diagram of the geological periods introduced in a previous article, I have represented all the three classes of Radiates, Polyps, Acalephs, and Echinoderms, as present on the first floor of our globe that was inhabited at all.  But it is only recently that positive proofs have been found of the existence of Acalephs or Jelly-Fishes, as they are called, at that early period.  Their very name indicates their delicate structure; and were there no remains preserved in the rocks of these soft, transparent creatures, it would yet be no evidence that they did not exist.  Fragile as they are, however, they have left here and there some faint record of themselves, and in the Museum at Carlsruhe, on a slab from Solenhofen, I have seen a very perfect outline of one which remains undescribed to this day.  This, however, does not carry them farther back than the Jurassic period, and it is only lately that I have satisfied myself that they not only existed, but were among the most numerous animals in the first representation of organic life.