Several species of this and the following genus, perfectly silicified, are found in the state of pebbles and boulders in the superficial soil of Antigua, and other islands of the West Indies, associated with the fossil palms, described in a former part of this work. Some of these corals are of great beauty, and polished sections exhibit the coralline structure most perfectly.[247] [247] In the "Spongitenkalk," at Nattheim, near Heidenheim, all the corals are replaced by chalcedony. Madrepora.--Polyparium arborescent or frondescent, porous, fixed; cells deep, with twelve rays, prominent, irregularly dispersed on the surface, and accumulated towards the terminations of the coral. The term madreporite, or fossil madrepore, was formerly applied to all the branched fossil corals with radiated cells, but is now restricted to those which possess the above characters. The recent common species, figured _Wond._ p. 620, will serve to illustrate this genus. The elevated, branched Madrepores, with minute polygonal cells having twelve rays, the lamellæ of which are denticulated, are termed Porites, and are frequent in the Silurian strata (_Sil. Syst._). Millepora. _Lign. 89._--Coral ramose; cells very minute, distinct, perpendicular to the surface, giving the interior a finely striated fracture, disposed irregularly. There are many fossil species of this genus, some of which are of considerable size. A small species from the mountain limestone is figured _Lign. 89, fig. 7_.[248] [248] Millepora, Pict. Atlas, pl. xl. _fig._ 6. Lithodendron. _Lign. 70, fig. 3._--Polyparium branched, formed of deep, cylindrical, elongated cells, which are terminal, and radiated, with a prominent central axis. Large masses of corals of this genus, composed of clusters of branches, are imbedded in the mountain limestone of Derbyshire, Yorkshire, &c.; and a few species occur in the Coralline Oolite; their general configuration will be understood by the figure _Lign. 70, fig. 3_; but in this specimen the margins of the cells are worn off, and do not present the original deeply excavated form.[249] [249] Lithodendron fasciculatum, Pict. Atlas, pl. xxxviii. There is a remarkable specimen of this coral in the Bristol Institution (of which a portion is now placed in the Museum of Practical Geology, in London), that was discovered by Mr. Samuel Stutchbury,[250] in a vein of hematitic iron ore. It is a large mass, in which the entire substance of the coral is transmuted into a metallic ore, forming one of the most interesting natural electrotypes I have ever seen. In this instance, a block of Lithodendron must have lain in a vein or fissure of the rock, and its animal membrane have resisted the action of the gaseous emanations, or mineral solutions, while the calcareous polypidom was dissolved, and the metallic matter deposited atom by atom, as in the case of pseudo-morphous crystals. [250] Now of Sydney, Australia.