The helmet Echinites, like the preceding, have given rise to innumerable siliceous casts, which are found associated with those of other forms in the Drift, on the ploughed lands, and among the shingle on the sea-shore; they are often placed as ornaments on the mantel-shelves of the cottagers. A flint cast of an Ananchyte, in which the plates were partially separated, is represented _Lign. 103, fig. 1_. The shells are sometimes filled with pyrites; and occasionally are found partially empty, with crystals of calc-spar symmetrically arranged on the inside of the shell, parallel with the rows of ambulacral pores. _Lign. 103, fig. 3_, is a remarkable example, in which flint occupies the base of the shell, while the upper surface is lined with crystals of calcareous spar. [Illustration: Lign. 107. Micraster (_Spatangus_) cor-anguinum. _Chalk. Lewes._ Fig. 1.--View from above, showing the petaloid ambulacra. 2.--View of the base, with the mouth. ] Micraster cor-anguinum (_Snake-heart_). _Lign. 107._--Of this genus there are many species in the Chalk. This type of Spatangidæ are more or less oval, elongated, and heart-shaped, wider before than behind, with a sulcus, or furrow, in front. The shell is fragile, and composed of large polygonal plates; the tubercles small and irregularly distributed; the spines are short. The mouth is transverse, situated anteriorly, and protected by a strong projection of the odd interambulacrum, which is named the lip. The vent is terminal, and placed above the margin. There are but four ambulacra, and these are incomplete, comparatively of small extent, and situated in deep furrows. A large and new species of Micraster (_M. cor-bovis_, of Prof. E. Forbes), from the Sussex Chalk, is figured in Dixon's _Fossils_, pl. xxiv. figs. 3, 4, p. 342. [Sidenote: TOXASTER COMPLANATUS.] [Illustration: Lign. 108. Toxaster complanatus. _Greensand. Switzerland._ Fig. 1.--Profile. 2.--View of the summit, showing the _Vent_ at the side; _e_. 3.--View of the base, displaying the situation of the mouth, and the union of the five _ambulacra_; their pores are not introduced in figs. 2 and 3. _a._ The narrow porous divisions of the shell, termed _Ambulacra_. _b._ Interambulacral spaces. _c._ _Areæ_, or spaces covered by the wide plates. _d._ The _Mouth_. _e._ The _Vent_, or _Outlet_. ]