THE AGE OF INVERTEBRATES OF THE SEA. 6i 



urchins. These merely put in an appearance in the Early 

 Cambrian, but become vastly multiplied in the Silurian, 

 where the stalked feather stars (Crinoids) (Fig. 46), seem to 

 have covered great areas of sea-bottom, and multiplied so 

 rapidly that thick sheets of limestone are largely made up of 



Fig. 46.— Modern Crinoid {Rhizocrinus Lqfotensis).—Mx.tx Sars. 



the fragments of their skeletons. The ordinary star-fishes 

 appear first in the Silurian (Fig. 47). The sea-urchins begin 

 in the Upper Silurian, the early species having numerous and 

 loosely attached plates, like some of those now found in the 

 deep sea ' (Fig. 48). 



* PaliTchinus. 



