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. [Illustration: 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. The common Echinus of our sea-coasts (_Echinus sphæra_), known by the name of Sea-egg, Sea-urchin, or Sea-hedgehog, presents the typical characters of this order of Radiata, which differs from the Crinoids and Star-fishes in the absence of arms. The calcareous envelope of the Echinus, or shell, as it is popularly called, is composed of many calcareous, polygonal plates, closely fitted to each other, and arranged in regular and elegant patterns; appearing in the globular and spherical kinds like the lines of the meridian on a globe. The plates are disposed in ten vertical series, united by serrated sutures, and form sections, into which the envelope or case very commonly separates upon the decay of the investing integuments. Of the ten bands five are large and five small. The large bands (_areæ_) are each composed of a double row of plates, ornamented with tubercles supporting large spines (_Lign. 100_). The five narrower bands consist of much smaller plates, and the spines are minute, or altogether wanting; hence these bands appear like avenues through the spinous tracks, and have therefore been fancifully termed ambulacra or walks. They are the equivalents of the grooves or furrows of the Star-fishes (_Lign. 108, a_.); and are traversed by numerous pores, for the exsertion of tubular feet, or tentacula. Besides these rows of minute openings, there are two principal apertures, the mouth and the outlet or vent; and also a few large pores, commonly five, on the summit, for the exclusion of the ova, and the free admission of water.