A species, in which the mouth is pentagonal, and the outlet on the margin, occurs in the Chalk-marl of Sussex, Dorset, and the Alps; a specimen is figured in [Lign. 106].
Lign. 106. Discoidea (Galeritcs) castanea. Chalk-marl, Dorset.
| Fig. | 1.— | Profile. The pores and plates of the ambulacra only are inserted in this figure. |
| 2.— | View from above. | |
| 3.— | The base, showing the central pentangular mouth, and the vent in the margin. |
The Clypeideæ differ from the tribe of echinites last described, in the ambulacra being petaloid, that is, of a leaf-like shape, and disposed in a stellated figure on the upper part of the shell. The ambulacra do not extend to the mouth. The shell is generally of a depressed form; and the petaloid ambulacra in many species appear like an elegant star, richly fretted, spread over the shell. There are numerous species of this type, both recent and fossil; many of the latter, being of a large size, are beautiful objects in a cabinet of petrifactions.
Clypeus sinuatus (Pict. Atlas, pl. liv. fig. 1).—Of this genus, which is the type of the tribe, a large species, C. sinuatus, is very common in the Oolite of Wilts, Gloucestershire, and Oxfordshire, and must be familiar to collectors. The shell is circular, and much depressed; and has five petaloid ambulacra: the odd interambulacral area forms a deep furrow in which the outlet is situated: the mouth is median, pentagonal, and surrounded by a strong margin. The coloured figures in Pict. Atlas, pl. liv. will enable the student to recognize these fossils without difficulty. Splendid specimens of an allied form (Clypeaster) occur in the tertiary limestone of Malta, (Pict. Atlas, pl. lvi. fig. 7,) and are not uncommon in collections.
Nucleolites (Wond. p. 328).—There is a small type belonging to this family, of which several species are so abundant in the Oolite, Greensand, and Chalk-marl, that a brief notice of their characters may be useful. The shell is oblong and inflated, rounded in front and flat behind. The pores are united by grooves; the outlet is in a deep furrow on the superior face; the mouth is sub-central. One species occurs in the Tertiary strata, and there is a recent species inhabits the seas of Australia. There are coloured figures of Nucleolites in Pict. Atlas, pl. liv. fig. 5, pl. lv. figs. 6, 8.
Spatangidæ.—In this tribe of echinites, the case is oblong or cordiform. The mouth, elongated transversely and destitute of proper jaws, is situated in front of the centre of the base, near the anterior border of the periphery. The outlet is towards the posterior margin. The tubercles and spines are very small. Four subdivisions are established, namely,—
1. Ananchytes.—A thick and oval shell; the ambulacra simple and converging towards the summit; the mouth transverse; the outlet is situated on the inferior face. (Lign. 104, fig. 2.)