FOSSJl STAR-FISHES AND SEA-URCHINS, 143 



geas of the present day, The Sea-urchins are more 

 abundant now than at any previous period in the 

 \vorld's history. They inhabit every sea, and almost 

 every depth in the seas. More than at any other 

 time, one modern group of them (the Echinoidea) now 

 merits the name of Echinodermata, or " Spiny-skinned," 

 given to the entire order, The common Sea-urchins, 



Fig. 118,— upper side of ditto. 



such as Echimis esctilenta or E. miliarisy are covered 

 with what are not inaptly called " spines." 



The Echinoidea are doubly important, on ac- 

 count of their numerical abundance and wide dis- 

 tribution in the seas of the present day, and their 

 great geological antiquity. Their general persistence 

 in the rocks of every geological epoch from the 

 Silurian up to our own is remarkable, and we find 



