For the convenience of study, the fossil Echini may be separated into three principal groups; viz. the Cidaritidæ, or Turban Echinites; the Clypeasteridæ, or Buckler-shaped Echinites; and the Spatangidæ, or Heart-shaped Echinites. [Sidenote: CIDARITIDÆ.] Cidaritidæ. _Turban Echinites._--In these sea-urchins the vent is situated on the summit of the shell, and is surrounded by five minute apertures for the exclusion of the ova. The mouth, or oral aperture, is placed directly opposite, in the middle of the base, or inferior surface. The mouth is large, and furnished with a powerful apparatus, armed with teeth; which is sometimes found fossil (see _fig._ 1, 3, _Lign. 101_). The structure of the animal, in consequence of the symmetrical position of the two chief outlets of the shells, is strictly radiated; that is, all the parts proceed from, or are arranged around one common centre. The tubercles are larger and fewer than in the other tribes of this family; they support long and powerful spines, which vary much in form and ornament in different species (see _Lign. 102_). The larger tubercles and spines are beset with smaller ones, disposed in regular series. 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.