The Turban Echinites are the most ancient types of the order, some forms appearing in the Carboniferous deposit. The species are very numerous. The echinites of this group are subdivided into four tribes:
1. Cidares, properly so called.—In these the tubercles are perforated; the ambulacra narrow, and beset with granular tubercles, and the two lines of pores are close together. The pseudopodia can be protruded to a great length, even beyond the spines, so as to reach objects the latter could not touch.
2. Echini, which differ from the above in the tubercles being imperforate, and the ambulacral area wide: the spines and tubes are of a moderate size.
3. Diademæ.—The tubercles are perforated, and the ambulacra wide and studded with large tubercles.
4. Saleniæ.—The tubercles are imperforate, and relatively large; the chief distinction is a solid ovarian disk on the summit, composed of several large flat plates, anchylosed together.
The case of the Turban Echinites is composed of twenty vertical series of plates, the ambulacra, or porous grooves, forming continuous bands from the summit to the mouth, which is armed with five angular teeth. This tribe comprises many of the most elegant fossil species; those which, from their shape and highly ornamented surface, have received the popular name of Fairy's night-caps. The genus Cidaris, which is characterized by perforated spinous tubercles, affords the most beautiful examples, and these are occasionally found with the spines in contact; a circumstance less rare than might be supposed, when the nature of the attachment of these organs is considered; for, upon the decomposition of the integument, and the ligaments which connect the spines with the tubercles in a living state, these appendages quickly fall off, even in recent specimens.
The interesting fossil figured [Lign. 100] (ante, [p. 311].), is a choice example of a Cidarite with the spines attached. This species (Hemicidaris crenularis, Agassiz) is said to be characteristic of the Upper Jura limestone of Switzerland, and was supposed to be identical with Mr. Parkinson's Cidaris parpillata var. (Pict. Atlas, pl. lvi. fig. 6), from Calne, in Wiltshire; but spines like those of [Lign. 100], do not occur in the English oolite. These spines are not homogeneous throughout; the central part appears to have been of a less dense tissue than the outer coat, as is shown in the fractured spine in the figure. This structure does not exist in the spines of the depressed Turban Echinites, but is stated by M. Agassiz to prevail in all the species of the genus Hemicidaris, of which the fossil figured in [Lign. 100] is the type.
Lign. 101. Cidarites from the Oolite and Chalk.