I22 HISTORICAL PALAONTOLOGY. 
another. The arms are typically five in number; but they 
generally subdivide at least once, sometimes twice, and they 
nee. 
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Pe 
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2 
i) 
Gag 
vidi 
(| 
meee’ 
yt) 2) 3) 
BS, 
ris(= 
a 
i 
i PU ise a 
Binge 
Gs 
STARR 
(TTe] [7 
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Dp. 
= isi, 
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: 
Fig. 62.—Upper Silurian Crinoids. a, Calyx and arms of Zucalyptocrinus polydacty- 
Zus, Wenlock Limestone ; 6, /chthyocrinus levis, Niagara Limestone, America; ¢, 
Taxocrinus 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 mznwtie 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, 
Lucalyptocrinus, Ichthyocrinus, Mariacrinus, FPeriechocrinus, 
Glyptocrinus, Crotalocrinus, and Ldriocrinus. 
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 (Planolites), which are 
