THE BRACHIOPODA 17 



closing the shell. Fifteen species of living lingulids are 

 known, and a very great number of fossil species. 



The resemblance of many fossil brachiopod shells to 

 those of living forms compels us to believe that the 

 former were also once parts of living organisms which 

 lived in much the same manner. Where therefore we 

 find brachiopod shells in a stratum we must conclude 

 that it was originally a deposit on the sea-bottom, and 

 we may even venture to estimate the depth below the 

 surface at which it was deposited from a consideration of 

 the abundance and generic identity of the brachiopods. 

 But the fossils also serve another very important 

 purpose. 



As long ago as 1688, the famous Dr. Robert Hooke 

 foresaw the possible utility of fossils as time-markers. 

 " However trivial a thing a rotten shell may appear to 

 some," he wrote, " yet these monuments of nature are 

 more certain tokens of antiquity than coins or medals . . . 

 and though it must be granted that it is very difficult to 

 read them and to raise a chronology out of them . . . 

 yet it is not impossible." It was William Smith who, at 

 the end of the eighteenth century, was first able to " raise 

 a chronology out of them," by showing that in the series 

 of strata that lie one upon the other in the Bath district 

 (and elsewhere) each division is distinguished by particular 

 species of fossils. 



Thus William Smith found that underneath the Corn- 

 brash were a series of beds of hard flaggy limestone, 

 which he termed the " Forest Marble " ; below these again 



