THE SILURIAN SYSTEM. 



677 



dwindled down to two analogues, which inhabit our existing seas. The animal occupied 

 a fixed position at the bottom of the ocean, like its modern representatives, or was attached 

 to floating pieces of wood, merely moving itself as far as it could reach by bending its very 

 flexible column, which was admirably adapted for this purpose. This stem, which may be 

 called the vertebral column, although the Encrinites are invertebrated, consists of a vast 

 number of ossicula, little bones or joints, with a central perforation, so as to admit of 

 being strung together when found detached. They commonly occur singly in the northern 

 counties, passing under the denominations of " wheel-stones," and " St. Cuthbert's beads," 

 from having been strung as beads, and formerly used as rosaries. Hence the lines in 

 Marmion : 



" On a rock by Lindisfcrn 

 St. Cuthbert sits, and toils to frame 

 The sea-born beads that bear his name." 



Dr. Mantell states that he has found these circular perforated ossicula, which had 

 been Avorn as ornaments, in tumuli of the ancient Britons ; and also, that the " channel 

 formed by the united ossicula of the column, has given rise to the curious fossils called 

 in Derbyshire screio or pulley stones, which are flint casts of these cavities. They occur 

 in the beds of chert which are interstratified with the mountain limestone ; the siliceous 

 matter, when fluid, filled up the channels, and invested the stems : the calcareous substance 

 has since been dissolved and removed, and solid cylinders of flint, resembling a pulley, 

 remain. In the quarries on Middleton Moor, near Cromford, where extensive beds of 



1. Fenestilla prisca. 



2. Fenestilla Milleri. 



3. Ketepora infundibulum. 



LUDLOW CORALS. 



4. Heteropora crassa. 



5. Stromatopora nummulitisimilis. 



6. Aulopora serpens. 



7. Cyathophyllum dianthus. 



limestone, composed of crinoideal remains, are worked for chimney-pieces and other 

 ornamental purposes, beautiful examples of these fossils may be obtained. The cavities 

 of the column and ossicula are often filled with white calcareous spar ; while the ground 

 of the marble is of a dark reddish-brown colour." Man now erects his mansion, and has 

 its apartments adorned with variegated slabs, little imagining that the component parts 



