Pentacrinites.—The description of the recent Pentacrinus caput-medusæ (ante, [p. 282].), illustrates the characters of the crinoideans whose fossil remains are so familiar to the palæontologist, under the name of Pentacrinites. In these animals the pieces composing the receptacle are firmly articulated together; the rays of the disk are fixed immediately to the summit of the column by special ossicula; and the stem is composed of angular pieces, which are generally pentagonal. The receptacle is small, and situated deep between the bases of the arms; it is closed above by an integument covered by minute plates or flat ossicles ([Lign. 94, fig. 2]). The fossil remains of several species are abundant in the Lias and Oolite of Dorsetshire, Somersetshire, &c. Slabs of limestone may be extracted with the surface covered with these crinoideans, spread out as if floating in their native element; very commonly they are transmuted into sulphuret of iron, or have a coating of brilliant pyrites.[282] The neighbourhoods of Lyme Regis, and Charmouth, are celebrated for these organic remains. A small specimen of the arms of a pentacrinite on Lias shale is figured in [Lign. 94, fig. 3].

[282] Pictorial Atlas, pl. li. lii.

The arms in many of the plumose pentacrinites are very long and thickly beset with side-arms, and minute pinnæ, all of which are composed of separate articulated ossicles, so that the number of bones in a single endo-skeleton of those crinoids amounts to from fifty to one hundred and fifty thousand distinct pieces. The Briarean Pentacrinite,[283] so named from its numerous tentacula, is literally a tuft of articulated processes, appearing like a delicate fibrous plume attached to a stem. The Pentacrinus Hiemeri is a beautiful example of this type of crinoids, of which there is a noble group, comprising upwards of thirty individuals, on a slab in the British Museum,[284] exposed on the surface of the stone in as perfect a state as if just dredged up from the bottom of the sea. The pentacrinites are for the most part entire; the peduncle being fixed, and the column extending upwards in gentle undulations, and supporting the receptacle, from which the arms are gracefully outspread in various attitudes. The structure of the receptacle, and of the arms, and the extreme delicacy of the finer tentacula made up of countless minute ossicula, are admirably shown in this unique and most instructive fossil.

[283] Pictorial Atlas, pl. xlvii. The Briarean Pentacrinite is fully illustrated and described in detail in Dr. Buckland's Bridge water Essay, p. 484.

[284] This species was named and figured by M. König in his "Icones Fossilium sectiles," pl. iii. fig. 29, in 1826. See Petrifactions, p. 88.

Lign. 94. Actinocrinites, or Nave Encrinites.

Fig.1.—Actinocrinus Parkinsoni. (Org. Rem. ii. pl. xvii. Pict.
Atlas, pl. li.)
2.—Section of an Actinocrinus, (Miller's Crinoideæ, pl. ii.)
a. Proboscideal protrusion of the plate and integument.
b. Sections of the folded or closed arms.
3.—Arms of a Pentacrinite, on Lias-shale; Lyme Regis.