The British species of fossil Crinoidea amount to more than two hundred, and when the great number of bones that enter into the composition of the skeleton of a single Pentacrinite or Encrinite is considered, the prodigious quantity of the fossil remains of these zoophytes in the ancient deposits may be readily conceived. Polished slices of the encrinital marbles of Derbyshire, and of the Lias limestones from Lyme Regis and Charmouth, should be obtained, as they show sections of the imbedded crinoidal stems and detached ossicula; and sometimes of the receptacles.
[CHAPTER IX.]
FOSSIL ECHINIDÆ, OR SEA-URCHINS.
Lign. 100. Turban Echinus, with its spines; 1/2 nat.
(Hemicidaris crenularis, Agassiz.)
Jura limestone.
The fossils we have now to examine are among the most familiar of the objects commonly known as petrifactions; for as the enveloping cases of the Echini possess considerable durability, they have served as moulds into which silex, calc-spar, limestone, pyrites, and other mineral substances, when in solution, or in a semi-fluid state, have percolated, and formed sharp and enduring casts, which exhibit the forms of the plates, and the disposition of the pores, striæ, &c. of the original structures.