4.--Portion of a Ventriculite; the stem towards the base is enveloped in flint, and three radicle processes spread from it into the surrounding chalk at _a_. ] These specimens demonstrate that all the flints referred to, have been moulded in the cavities of cyathiform zoophytes; and that their diversity of figure has arisen from the amount of silex that happened to permeate the organism; if but a small proportion of silica in solution was present, then flints, like _fig._ 4, were produced; if the quantity were sufficient to fill up a considerable part of the tissues of the original, fungiform flints, as _Lign. 80, fig. 3_, and _Lign. 81, fig. 3_, were the result. The disciform flints originated from the expanded examples, _Lign._ 81, _figs._ 1 and 2: and when the silex was insufficient to silicify the entire zoophyte, an annular flint, resembling a quoit, was formed. [Illustration: Lign. 82. Portions of Ventriculites; 1/4 _nat._ _Chalk, near Lewes._ Fig. 1.--A specimen, in which the middle is inclosed in a flint _c_; the external structure of the Ventriculite is seen at _a_, expanded on the chalk; and the pedicle with its roots is exposed at _b_. The figures are one-fourth the size of the originals. 2, 3, 5.--Chalk specimens, showing the external structure of stems of Ventriculites. 4.--A siliceous cast of the cavity of a Ventriculite covered with papillæ, moulded in the orifices of the cells. ] The form of the original was evidently that of a hollow inverted cone, terminating in a point at the base, which was attached by fibrous rootlets to other bodies. The outer integument was reticulated, that is, disposed in meshes, like net-work; and the inner surface studded with regular openings, apparently the orifices of tubular cells. The substance of the mass appears to have been sufficiently flexible to expand and contract without laceration. This opinion is based on the fact, that in many specimens the zoophyte is a nearly flat circular disc (_Lign. 81, figs. 1, 2_); and in others a subcylindrical pouch. In the former state the outer reticulated structure is elongated, while in the latter, it is corrugated; hence I am led to conclude that the original possessed a common irritability, and was able to contract and expand like many of the flexible polypiaria. The openings on the inner surface are cylindrical, and very regular; the flints often present sharp casts of them, which appear like rows of minute pillars. When the flint filling up the cavity of a Ventriculite can be extracted, it is a solid cone, studded with papillæ, the casts of the cells, as in _Lign. 82, fig. 4_.