In the flints, the substance of the Ventriculites is generally as translucent as that of the Choanites, and defined by its rich purple, sienna, or grey colour;[224] but towards the base and margin it is more or less calcareous; and in many examples the whole, or a large portion of the zoophyte, is in this state. But this fact does not invalidate the inference that the original was flexible; for in these instances the tissues may have been immersed in fluid chalk before their envelopment in flint,[225] The chalk specimens are commonly as friable and earthy as the surrounding stone, from which they are distinguishable by their ochreous colour. [224] Pict. Atlas, pl. xlv. _fig._ 9, represents a beautiful transverse section of the lower part of a Ventriculite in flint, richly coloured. Pl. xliii. _fig._ 16, is a pebble containing the base of a Ventriculite; the orifices on the top have been produced by the transit of the radicle processes; for the fossil is drawn in an inverted position, a common error before the origin of these flints was ascertained. [225] A piece of sponge dipped in liquid plaster of Paris, and afterwards inclosed in a transparent substance, as glass, would present such an appearance. The stain always observable in the tissues of the chalk Ventriculites and other zoophytes, while the surrounding white limestone is uncoloured, may be explained by the chemical changes to which the decomposition of animal matter under such circumstances would give rise. If sulphuretted hydrogen were evolved from the putrifying zoophytes imbedded in calcareous mud containing iron in solution, the sulphur would enter into combination with the iron, the hydrogen escape, and a sulphate or sulphuret of iron be deposited, atom by atom, and thus impart colour and permanence of form to the original. When the inclosed organisms in the flint nodules have perished, chalcedony, quartz crystals, or crystallized pyrites, sometimes of great beauty, are found occupying the cavities; in short, numerous modifications of the petrifactive process are beautifully exhibited in these common, but highly interesting, cretaceous fossils. The species to which the previous remarks more immediately refer, is named _Ventriculites radiatus_; from the radiated appearance of the external integument; some of the expanded specimens are more than one foot in diameter.[226] [226] The reader interested in the history of these objects should consult Foss. South Downs, p. 167, plates x, xi, xii. xiii. xiv. A memoir by the Author on these fossils, under the name of _Alcyonium chonoides_, with four beautiful plates, was published in the Linnæan Transactions, vol. xi. 1821. The _Ventriculites_ are the only organic remains figured in Conybeare and Phillips's Geology of England and Wales, p. 76. [Illustration: Lign. 83. Ventriculites alcyonoides. _in Flint. Lewes._ _Ocellaria inclusa. König._ Fig. 1.--Portion of the surface of fig. 2, magnified.