| Fig. | 1.— | Spine of Acrocidaris nobilis (Agassiz). |
| 2.— | —— Cidaris cucumifera. | |
| 3.— | —— —— stemmacantha. | |
| 4.— | —— —— meandrina. | |
| 5.— | —— —— spinosa. | |
| 6.— | —— —— clunifera. | |
| 7.— | —— —— sceptrifera. | |
| 8.— | —— —— Parkinsoni. | |
| 9.— | —— Hemicidaris intermedia. |
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.
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.
CIDARITIDÆ OF THE PALÆOZOIC ROCKS.