144 OUR COMMON BRITISH FOSSILS, 



their species and types increasing in number in pro- 

 portion as we approach the present epoch. The 

 common sea-urchin {Echinus miliaris) is a familiar 

 example. It well deserves its name, for, when alive, 

 it is so thickly covered with spines as to greatly 

 resemble the common hedgehog ; when dead, these 

 spines peel off, and the surface is then seen covered 

 with minute knobs or tubercles, to each of which a 

 movable spine was attached, on the principle of the 

 ball-and-socket joint. The shell is composed of car- 

 bonate of lime, and is made up of an innumerable 

 number of separate pieces, all of which are mosaicked 

 together. No fewer than six hundred of these go to 

 make up the entire " test," as the shell is technically 

 called. And yet, although in the adult state it may 

 be several inches in diameter, the shell has not been 

 moulted since the animal was small. The membrane 

 lining the exterior of the test or shell secretes the 

 carbonate of lime diffused through the sea-water. As 

 the membrane is inserted between every one of the 

 six hundred and more plates, it is able to add lime 

 along the edges of each, and thus the whole structure 

 grows out uniformly and symmetrically, almost like 

 the expanding of a bubble when blown out. A more 

 beautiful architectural contrivance could not be 

 imagined than is thus furnished by this insignificant 

 creature ! 



Take one of the rounded tests we may pick up 

 at the seaside, out of which the animal has been 



