The Medals of Creation, Volumes 1 and 2 / First Lessons in Geology and the Study of Organic Remains

by Gideon Algernon Mantell

2. Spatangus.—This name is now restricted to those Spatangidæ in which the ambulacra are petaloid, the external row of pores slightly elongated, and the inner rows round.

3. Micraster.—By this term are now distinguished the Spatangidæ which have the ambulacra depressed, and the shell cordiform. The pores of the even ambulacra are united by a furrow. The vent is on the posterior face. The common chalk Spatangus belongs to this genus. ([Lign. 107.])

4. Holaster.—The shells are heart-shaped; the ambulacra simple, and converging towards the summit; the mouth is transverse; the outlet is within the posterior face.

ANANCHYTES OVATUS.

Ananchytes ovatus. [Lign. 104.] fig. 2.—These sea urchins are among the most characteristic of the fossils of the Upper Chalk, and are peculiar to the Cretaceous formation. They are readily distinguished by their elevated helmet-like form, and by the transverse mouth and oblong outlet situated on the inferior face of the flat base, and towards the margin. ([Lign. 104. fig. 2^a.])

The vernacular names of "Shepherd's Crown," and "Fairy Loaf," indicate the form of these abundant fossils. The shell is oval in its longest diameter; flat, or nearly so, below; and rounded, conical, and somewhat laterally compressed towards the summit. The ambulacra are five, between double lines of pores; the tubercles are minute and scattered; the substance of the shell is of great thickness. More than twenty species of the genus are known.

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.

Lign. 107. Micraster (Spatangus) cor-anguinum.
Chalk. Lewes.