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HISTORICAL PALAEONTOLOGY. 



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

 generally subdivide at least once, sometimes twice, and they 



Fig 62. Upper Silurian Crinoids. a, Calyx and arms of Encalyptocrinus polydacty- 

 /HS, Wenlock Limestone ; b, Ichthyocrinns Icevis, Niagara Limestone, America ; c, 

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



are furnished with similar but more slender lateral branches 

 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 minutia of structure 

 by which the innumerable parts entering into the composition 

 of a single Crinoid are adapted for their proper purposes 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, 

 Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus, 

 Glyptocrinus, Crotalocrinus, and Edriocrinus. 



The tracks and burrows of Annelides are as abundant in 

 the Upper Silurian strata as in older deposits, and have just 

 as commonly been regarded as plants. The most abundant 

 forms are the cylindrical, twisted bodies (Planohtes), which are 



