Spines of Cidarites. _Lign. 102._--Allusion has been made to the immense numbers of the spines of two or three kinds of Cidarites that occur in the oolitic limestones of certain localities. The spines of other species and genera abound in the Chalk, Greensand, &c.; occurring detached and intermingled with corals, shells, and the usual fossils of those deposits. There is great variety in the form, size, and sculpture of these organs. In the subjoined _Lign. 102_, a few distinct kinds are represented.
[Illustration: Lign. 102. Fossil Spikes of Cidarites.
Fig. 1.--Spine of Acrocidaris nobilis (_Agassiz_).
Flint Casts of Turban Echinites. _Lign. 103._--The siliceous casts of the shells of the Turban Echinites are interesting objects, for they are often beautiful models of the interior. A specimen of this kind is represented in _Lign. 101, fig. 5_. Casts of the larger Cidarites are often seen on the ploughed lands of the South Downs, in beds of gravel, and among the shingle on the sea-shore of chalk districts; appearing as flattened spherical bodies, with a circular protuberance at each pole, and vertical rows of nodular projections. Impressions of the external surface of the cases are also frequent on chalk-flints, and exhibit exquisite casts, in intaglio, of the mamillated tubercles, and ambulacral grooves and pores.
[Illustration: Lign. 103. Echinital Remains in Flint. _Chalk. Lewes._
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.