As in the more recent forms, these Cidarites are separable into two groups or families; one in which the spiniferous tubercles are imperforate as in the Echinus (_Palæchinidæ_ of M'Coy); the other with numerous small secondary tubercles and a few large primary ones, perforated for the ligament of the spine as in Cidaris (the _Archæocidaridæ_ of M'Coy).[301] [301] Prof. Sedgwick's "British Palæozoic Fossils," p. 124. Clypeastridæ.--The shell in this family of sea-urchins, is oblong or rounded; the mouth is of an angular form, and situated in the middle of the base or inferior face; it is furnished with well-developed dental organs. The outlet is distant from the summit. The tubercles are mere granulations, and the spines proportionally small. This group is subdivided into two tribes: the Galeritidæ (_helmet-like_), and the Clypeideæ (_buckler-like_). Galerites albo-galerus. _Lign. 104._--The tribe of which this genus is the type has the shell inflated, orbicular, oblong, or pentangular. The ambulacra are simple, never petaloid; the poriferous zones extend uninterruptedly from the summit to the mouth. In the species figured _Lign. 104, fig. 1_, the shell is of a conical form, in some varieties subpentagonal; narrowest at the hinder part. The mouth is of a decagonal shape, and armed with teeth: it is situated in the centre of the base (_Lign. 104, fig, 1a_); the outlet is near the posterior margin of the base. The surface of the shell is covered with granulations irregularly distributed. This species, which received the name of _albo-galerus_, from its fancied resemblance to the white conical caps of the priests of Jupiter, occurs in great numbers and perfection in the Kentish chalk; it is less common in that of Sussex. Siliceous casts of the shell are constantly found among the drift and gravel, and on the ploughed lands of chalk districts; they are popularly termed "_sugar-loaves_." The specimens obtained from the chalk, when filled with flint, yield exquisite casts, if the shell be dissolved in dilute hydrochloric acid; by this means the form of the plates, and casts of the minutest ambulacral pores are obtained. [Sidenote: ECHINITES FROM THE CHALK.] [Illustration: Lign. 104. Echinites from the Chalk. _Lewes._ Fig. 1.--Galerites albo-galerus: _nat._ 1_a_.--Base of the same, with the five teeth. 2.--Ananchytes ovatus: 1/2 _nat._ 2_a_.--Base of the same. ] [Illustration: Lign. 105. Holectypus (_Galerites_) inflatus. _Kimmeridge Clay, Switzerland._ The left-hand figure shows the summit; the middle figure a profile; and the right hand, a view of the base, with the mouth in the centre, and the outlet towards the margin. (_M. Agassiz_).]