Our previous examination of the specimen showed that the flint had once been in a fluid state, and had consolidated in a sea inhabited by shells, echini, fishes, corals, sponges, and other zoophytes; and the appearance of the fractured end (_Plate I, c_), indicated that some organic body had formed the nucleus of the pebble, and that traces of the structure of the original still remained. To ascertain if this inference is correct, it will be necessary to divide the stone in a longitudinal direction--but I will first strike off a small fragment, and examine it by the aid of a microscope. [Sidenote: FOSSIL ANIMALCULES.] [Illustration: Lign. 10:--Fossil animalcules (_Xanthidia_) in Flint.] By a sharp blow of a hammer, a very thin and minute portion of the flint has been detached (see _Lign. 10, fig. 1_); it is translucent, and when held between the eye and a strong light, appears like a slice of horn; and a few extremely minute specks may with difficulty be detected. Under the microscope, five of these almost invisible points are well defined, and present a radiated appearance (see _fig. 3_); but I will substitute a higher power, and lo! they are seen to be distinct globular or spherical bodies beset with spines (_fig. 3_); and with a still more powerful lens, one which magnifies many hundred times, their nature is completely displayed. The whole five possess this general character--a central globular case or shell, from which radiate tubes or hollow spines, that terminate in fringed or divided extremities (_figs. 4, 5, 6_); but these bodies differ from each other in the relative proportions of the shell and spines, and in the number, shape, and length of the tubular appendages. The group, in short, is separable into three distinct species, of the same kind of fossil remains; and several other varieties occur in the chalk and flint. . [Sidenote: XANTHIDIA IN FLINT.] [Illustration: Lign. 11:--_Xanthidium palmatum_ in flint: highly magnified.] But what are these bodies?--They are the durable cases of animalcules, many species of which swarm in our seas, and are so minute, that thousands may be contained in a drop of water! In a living state, the case is flexible and filled with a granular jelly, which is the soft body of the animalcule, and the tubes and the outer surface are invested with a similar substance. After death the soft parts dissolve; but the case and its spines often remain unchanged.