1 96 GEOLOGY. 



beautiful of the corals is given in Fig. 104, the chain cor- 

 al, the specimen pictured being from the cliff limestone 



Fig. 104. 



in Iowa. The Echinoderms styled crinoids, of which 4 

 and 5, Fig. 103, are specimens, stand 

 on stems or peduncles. In Fig. 105 

 is represented the base of the frame- 

 work of one of these animals. To 

 the central piece was fastened the 

 peduncle, and the branching arms 

 of the animal were jointed to the 

 side pieces. The fragments of the 

 stems of these animals found in the 



rocks were used as rosaries in the Middle Ages. They 



were called St. Cuthbert's beads, and are thus noticed by 



Sir Walter Scott in his Marmion : 



" On a rock by Lindisfarn 

 St. Cuthbert sits, and tries to frame 

 The sea-born beads that bear his name." 



286. Mollusks. In Fig. 106 you have specimens of the 

 shells of some of the varieties of Silurian mollusks. There 

 are three great classes of mollusks. 1. The Cephalopoda, 



