From the earliest ages the corals play a conspicuous part in fossil history; and as in our days we find them encircling islands and fringing continents with huge ramparts of limestone, so many an ancient reef, now far inland, and raised several thousand feet above the level of the sea, bears witness to the vast terrestrial changes that have taken place since it was first piled up by the growth of countless zoophytes. [Illustration: SIPHONIA COSTATA—FOSSIL SPONGE (GREEN SAND, WARMINSTER).] With regard to the dimensions of the fossil corals we do not find that any of them exceeded in size their modern relatives; but their construction was widely different. The fossil sponges of the primitive seas are likewise very unlike those of the present day. Thus in all the ancient strata we find abundant spongidæ with a stony skeleton, while all the modern sponges possess a horny frame. The Petrospongidæ, or stone sponges, which have long since disappeared, are frequently shapeless masses; but a large number are cup-shaped, with a central tubular cavity, lined, as well as the outer surface, with pores more or less regularly arranged. [Illustration: ENCRINUS LILIIFORMIS. (Muschelkalk, Germany.) ] [Illustration: PENTACRINUS BRIAREUS.] The Crinoids, or Sea-lilies, now almost entirely extinct, were extremely common in the primeval seas. Unlike our modern sea-stars, to which they are allies, they did not move about freely from place to place, but were affixed, like flowers, to a slender flexible stalk, composed of numerous calcareous joints connected together by a fleshy coat. The Carboniferous Mountain Limestone is loaded with their remains, and the _Encrinus liliiformis_ is one of the leading fossils of the Muschelkalk of the Triassic group. The _Pentacrinus briareus_ is of more modern date, and occurs in tangled masses, forming thin beds of considerable extent in the Lower Lias. This beautiful Crinoid, with its innumerable tentacular arms, appears to have been frequently attached to the drift wood of the Liassic sea, like the floating barnacles of the present day. In the still more recent Chalk group is found a remarkable form of star-fish, the _Marsupites ornatus_, which resembles in all respects the Crinoids, except that it is not and never was provided with a stem. It seems to have been rolled lazily to and fro, by the influence of the waves, at the bottom of the sea, and to have been anchored in its place by the action of gravity alone. [Illustration: MARSUPITES ORNATUS. CHALK.]