These petrifactions present considerable variety in form, and in the markings on their articulating surfaces, which are often radiated and sculptured in floriform and stellular figures (_Lign. 90, fig. 7, 9_, and _Lign. 91, fig. 3, 4_). The central perforation is small in some species, and large and pentagonal in others. The ossicula of the Encrinites often vary in size in the same column, being circular and elliptical, and thick or thin, alternately, as in the upper part of the column of the Lily Encrinite, _Lign. 91, fig. 6_; by which great flexibility and freedom of motion were obtained. The pentagonal stems also display many modifications; some have five, others but four sides (_Lign._ 90, _fig._ 6, 8, 10, and 91, _fig._ 7, 9); in some the angles are acute, in others rounded. Pulley-stones. _Lign. 90, fig. 1._--The circular, or pentagonal channel formed by the united ossicula of the column, has given rise to the curious fossils called in Derbyshire the Screw, or Pulley-stones, which are flint casts of those cavities that occur in the beds of chert, interstratified with the mountain limestone. The siliceous matter, when fluid, must have filled up the channel and invested the stem: the original calcareous ossicles have since been dissolved, and the casts, now solid cylinders of flint, resembling a pulley, remain. The masses of chert are often impressed with the ornamented articulating surfaces of the trochites. In the quarries on Middleton Moor, near Cromford, Derbyshire, where extensive beds of limestone composed of crinoideal remains are worked for chimney-pieces and other ornamental purposes, beautiful examples of these fossils may be obtained.[267] The cavities of the column and ossicles are often filled with white calcareous spar, while the ground of the marble is of a dark reddish brown colour; in other varieties of the Derbyshire encrinital limestones, the substance of the fossils is white, and the ground dark grey or brown,[268] A slab of this marble, with portions of columns lying in relief, and a polished section showing the inclosed entrochites, are figured _Wond._ p. 650.[269] [267] See Excursions around Matlock, Part IV. of this work. [268] Pict. Atlas, pl. xlix. figs. 1, 3, 6. [269] Upwards of 80 figures of Encrinital remains are given in Pict. Atlas, pl. xlvii. [Illustration: Lign. 91. Illustrations of Crinoidea. Fig. 1.--Group of Living Comatulæ; _nat._ (_Dr. Thompson._) 1_a_.--A single Comatula of the same group, magnified. 2.--Pentremites pyriformis. _Silurian, Ohio._ 3, 4.--Surfaces of two pentapetalous ossicula. 5.--A single Ossicle of an Apiocrinus. _Chalk._ 6.--Encrinus liliiformis; the receptacle with part of the column attached; 1/2 _nat._ _From Brunswick._ 7.--Pentacrinital stem with four angles. 8.--Entrochite, composed of five smooth ossicles. 9.--Quadrangular stem of seven ossicles.