Holectypus (_Galerites_) inflatus. _Lign. 105._--In certain kinds of Galerites, the shell is strengthened internally by five strong ribs or projections, which of course leave corresponding deep furrows or channels on the flint casts moulded in them; such fossils are not numerous on the ploughed lands of the South Downs. These echinites are placed by M. Desor in the genus of which an example is figured in _Lign. 105_. The shell is hemispherical, and circular; the base flat; the tubercles are disposed in series; the inside of the case is supported by ribs. [Sidenote: DISCOIDEA CASTANEA.] Discoidea (_Galerites_) castanea. _Lign. 106._--The Galeritidæ, which have a polygonal mouth, with the tubercles disposed in vertical rows from the summit to the centre of the base, as in the Cidarites, instead of being uniformly spread over the surface, as in _G. albo-galerus_, are placed in the genus _Discoidea_, by M. Agassiz. 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_. [Illustration: 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.