ACTINOCRINUS. CYATHOCRINUS.
Actinocrinus (Nave Encrinite). [Lign. 94, fig. 1, 2.] (Wond. p. 585. Bd. pl. xlvii.)—The column is formed of numerous round ossicula, possesses side-arms, and is fixed by root-like processes. The receptacle is of an irregular subspherical form, the arms passing off at right angles like the spokes from the nave of a wheel; hence the name. The base is composed of three plates which support five hexagonals and one pentagonal, and on these are three other series; from the upper margin of the last, five arms are given off. The receptacle is closed by a dome-shaped tesselated covering ([Lign. 94, fig. 2]), having on one side the opening for the mouth and vent. The specimen, fig. 1, is a receptacle without a stem, and with but a few joints of the arms; this is the usual state in which these fossils are obtained. Fig. 2 is an Actinocrinite in which part of the brim of the receptacle and of the arms has been removed in front, leaving a portion of the coalesced arms partially surrounding the proboscideal plated integument that covers the upper part of the receptacle; the figure is from Mr. Miller's work, and is introduced to illustrate the structure of these crinoideans.[285]
[285] Miller's Crinoidea, fig. N. pp. 98-100.
The external surface of the plates of the receptacle in the Actinocrinites, is generally covered with radiated markings and ridges, so that detached plates have been mistaken for those of Marsupites; see the restored figure of a Nave Encrinite, Wond. p. 654. In some species the receptacle is richly ornamented; but in the specimen figured, [Lign. 94, fig. 1], the surface has been defaced in consequence of the fossil having been immersed in strong acid.
Cyathocrinus (Cup-like Encrinite). [Lign. 95], and [Lign. 92, fig. 2].—The column is formed of round, depressed, perforated joints, articulating by radiated surfaces; pentagonal near the summit. The receptacle is composed of five pieces, succeeded by two successive series of five plates, with intervening plates, and supports five bifurcating, radiated arms.
Lign. 95. Cyathocrinites planus; nat.
Mountain Limestone, Clevedon.
The Encrinites of this genus have a light and elegant appearance: the forms of the plates composing the pelvis, and of the ossicula of the arms, are shown in [Lign. 92, fig. 2]; which represents a specimen of C. planus, from the magnesian limestone of Somersetshire; a beautiful example of the same species is figured in [Lign. 95]. The receptacle resembles in shape a depressed vase; its upper part was probably covered by a plated integument, having an aperture in the centre as in the Actinocrinites. Cyathocrinites occur in the Silurian, Devonian, and Carboniferous formations.[286]
[286] A figure of Cyathocrinus rugosus is given in Pict. Atlas, pl. xlix. fig. 4. The same work contains coloured figures of Pentacrinus scalaris (Goldfuss), pl. xlvii. figs. 57, 64, 66; Pentacrinus basaltiformis, pl. xlvii. fig. 47.