A general idea of the nature of the compound coral-zoophytes may be obtained by the examination of the common _Flustra_ or Sea-mat. This form of polyparia, when taken out of the water, appears to the naked eye like a patch of fine varnished net-work, adhering to a piece of sea-weed or stone; when viewed with a magnifying lens of moderate power, the surface is found beset with pores regularly disposed: and if the Flustra be examined while immersed in sea-water, its surface is seen to be invested by a gelatinous substance, and every pore is the aperture of a cell, whence issues a tube fringed at the extremity with long tentacula or feelers. These expand, then suddenly contract, withdraw into the cell, and again issue forth: the whole surface of the Flustra being studded with the hydra-like animalcules; each enjoying a distinct existence, the entire group being united by one common integument or calcareous frame-work. When the Flustra is exposed to the air, the polypes soon perish, the animal matter rapidly decomposes, and the calcareous lace-like skeleton alone remains. In the larger and more compact corals the phenomena are similar, differing only in degree. In a fossil state, the durable remains of the corals consist for the most part of the calcareous frame-work (or polyparium, as it is termed), which often possesses a crystalline structure; and in some instances is completely transmuted into silex, as in specimens from Antigua, the Falls of the Ohio, and from Tisbury in Wiltshire. (See Plate XXXVIII. figs. 12, 13.) I must refer to the Wonders of Geology for a more extended notice of fossil corals, and other zoophytes, and will only add that the calcareous and siliceous spines or spicula, not only of sponges, but also of Gorgoniæ, and other corals, are often met with in a fossil state.[106] [Footnote 106: See Wonders of Geology, vol. ii. Lect. vi. p. 634.] IX. Cuvier's Discoveries. _The Fossil Quadrupeds of Montmartre._ (Plate LXXII.)--The Palæotheria, Anoplotheria, and other genera of extinct quadrupeds related to the _Tapir_, whose remains were first noticed in the gypseous limestone of Montmartre, near Paris, and which have since been found in many other localities of the same strata, are familiar to every one, from the just celebrity attached to the labours of the illustrious Cuvier, who restored as it were these lost denizens of an earlier world, in their native character and forms, and distinguished them by names long since become classical in the sciences which treat of the ancient history of the earth and its inhabitants.