l6o OUR COMMON BRITISH FOSSILS. 



minimizing the importance of the markings left by 

 other animals. 



If subsidence of certain parts of the sea-floor were 

 not accepted as a geological and geographical fact, 

 sea-worms would prove it more than any other 

 animals. For when we find such rocks as the Long- 

 mynds, composed of strata all more or less of a 

 similar physical character and composition, on whose 

 upper surfaces are innumerable tracks of sea-worms 

 for at least one mile in vertical thickness, no other 

 theory could account for the conditions under which 

 they had been formed than that which declares the 

 sea-bed was slowly subsiding, at about the same rate 

 that the sediments were accumulating. Moreover, 

 the same worm-tracks more or less indicate the depth 

 at which such deposits were formed, for we never find 

 these markings in strata of deep-sea origin ; and the 

 supplementary evidence of the ripple-marks so fre- 

 quently occurring in worm-tracked strata, is confirma- 

 tory on this point. 



Geology has little or nothing to say concerning 

 terrestrial worms, unless it refers to their physical 

 action in modern times. I am not aware of a 

 single species which can be safely referred to the 

 same habitats as our common earth-worms, although 

 I have little doubt this class has been in existence, 

 perhaps during the entire Tertiary period, if not 

 longer. I have therefore only to do with sea- 

 worms. These, as we now know them, may be 



