148 OUR COMMON BRITISH FOSSILS. 



the fossil Cidaridce resemble species now living iii 

 tropical and subtropical seas. The "cake-urchins," 

 of which our recent British species of Spatangus is a 

 well-known example, date from the Cretaceous, or 

 Chalk period ; and the fossils are so common as to 

 have obtained the popular name of " hearts " in chalk 

 districts. In number of species, however, and variety 

 of external form, these Echinoidea are most abundant 

 in Tertiary strata. It is a peculiar law in the history 

 of a race of organic beings, that they have a period 

 of introduction ; one when they reach their maximum, 

 both numerically and in variety of species ; and 

 another when these drop off one by one, and the race 

 becomes extinct. We then find that the functions 

 they performed are taken up by some other kindred 

 ^roup of animals, which, as a rule, is more highly 

 endowed and specialized, and so its members have 

 been able to thrust aside and extinguish their older 

 comrades ; just as British weeds are now supplanting 

 the native plants of New Zealand and elsewhere. 



The nervous system in a modern sea-urchin is 

 arranged round the mouth, which is furnished with five 

 hard calcareous teeth, to enable it to triturate its food. 

 These teeth are worked by muscles, through loops, 

 and the whole can be removed as easily as an artificial 

 set of teeth. In this state the mechanism goes by the 

 tiame of " Aristotle's lantern," and the seaside picker- 

 up of " unconsidered trifles " frequently finds it lying 

 by itself after the more fragile test has been broken to 



