[Illustration: Lign. 103. Echinital Remains in Flint. _Chalk. Lewes._ (_One-third the natural size._) Fig. 1.--Cast of an Ananchyte, showing the form of the plates. 2.--Imprint of a segment of a Cidarite on a pebble. _a._--One of the impressions of a spinous tubercle: _nat._ 3.--Portion of an Ananchyte, having the cavity of the shell covered at the bottom with flint, and lined above with crystals of carbonate of lime. ] A fragment of a flint, impressed by a portion of a Cidarite, is represented _Lign. 103, fig. 2_. The perforations around the imprint indicate tubular cavities in the flinty formed by the spines, and show that these processes were attached to the shell when the latter was enveloped by the fluid silex; the case and the spines having since perished. But in the Chalk, exquisite specimens of Cidarites occur with the case perfect, and filled with flint: examples of this kind are often attached to a nodule by the slender column of silex that fills up the aperture of the shell. The mineralized condition of the originally friable calcareous cases of Cidarites and other Echini, is worthy of attention: for whether the shell or spines be imbedded in chalk, flint, or pyrites, if the structure and form remain, the constituent substance is invariably opaque crystallized carbonate of lime, having an oblique fracture. As this conversion of a crustaceous envelope into calc-spar is constant, it has probably resulted from the peculiar nature of the original animal structures; but the cause of such transmutation is unknown. [Sidenote: CIDARITIDÆ OF THE PALÆOZOIC ROCKS.] Cidaritidæ of the Palæozoic Rocks.--Three genera of this family, comprising several species, have been discovered in the carboniferous limestone of Northumberland and Ireland. One genus is undistinguishable from Cidaris; and the species are placed under that name in Mr. Morris's Cat. Brit. Foss. These fossils have been figured and described by Prof. John Phillips, and Col. Portlock. Prof. M'Coy, with his accustomed penetration and sagacity, has ascertained, that notwithstanding the general resemblance between the Cidarites of the secondary and those of the palæozoic formations, the latter are constructed on an entirely different plan. In the turban echini of the secondary, tertiary, and modern seas, the interambulacral plates always consist of _two_ rows; but in the palæozoic Cidarites there are _three_ or some greater odd number of these plates. This eminent naturalist, therefore, places the earliest type of Echinidæ at present known in the order _Perischoechinidæ_. The case is spheroidal, formed of more than twenty rows of plates; five ambulacra composed of two rows of pentagonal plates each; rows of interambulacral plates, three, five, or more, terminating dorsally in five large pentagonal ovarian plates.