124 



HISTORICAL PALEONTOLOGY. 



more loosely connected by a leathery integument. From the 

 upper surface of the body, round its margin, springs a series 

 of longer or shorter flexible processes, composed of innu- 

 merable calcareous joints or pieces, movably united with one 

 another. The arms are typically five in number; but they 

 generally subdivide at least once, sometimes twice, and they 

 are furnished with similar but more slender lateral branches 



Fig. G2. Upper Silurian Crinoids. , Calyx and arms of Eucalyptocrinua polydac- 

 tylus, Wenlock Limestone ; b, Ichthyocrinus Icevis, Niagara Limestone, America ; c, 

 Taxocrinus tuberculatus, Wenlock Limestone. (After M'Coy and Hall.) 



or " pinnules, " thus giving rise to a crown of delicate feathery 

 plumes. The " column " is the stem by which the animal is 

 attached permanently to the bottom of the sea; and it is com- 

 posed of numerous separate plates, so jointed together that 

 whilst the amount of movement between any two pieces must 

 be very limited, the entire column acquires more or less flexi- 

 bility, allowing the organism as a whole to wave backwards and 

 forwards on its stalk. Into the exquisite minutice of structure 

 by which the innumerable parts entering into the composition 

 of a single Crinoid are adapted for their proper purpose in 

 the economy of the animal, it is impossible to enter here. No 

 period, as before said, has yielded examples of greater beauty 

 than the Upper Silurian, the principal genera represented 

 being Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus, 



