es 
FOSSIL CRINOIDEA. 285 
are often radiated and sculptured in floriform and stellular 
' 
figures (Lign. 90, fig. 7, 9, and Lign. 91, fig. 3,4). The cen- 
tral perforation is small in some species, and large and penta- 
_gonal in others. The ossicula of the Encrinites often vary 
p 
_ 
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 flexi- 
bility and freedom of motion were obtained. 
The pentagonal stems also display many modifications ; 
some have five, others but four sides (Lign. 90, jig. 6, 8, 10, 
and 91, fig. 7,9); in some the angles are acute, in others 
rounded. 
Puttey-stones. Lign. 90, fig. 1.—The circular, or pen- 
tagonal 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 cavi- 
ties 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, Der- 
byshire, where extensive beds of limestone composed of cri- 
noideal remains are worked for chimney-pieces and other 
ornamental purposes, beautiful examples of these fossils may 
be obtained.* 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 sub- 
stance of the fossils is white, and the ground dark grey or 
brown.t A slab of this marble, with portions of columns 
* See Excursions around Matlock, Part IV. of this work. 
+ Pict. Atlas, pl. xlix. figs. 1, 3, 6. 
