VIII 

 THE GRAPTOLITES AND CORALS 



THE Graptolites are an extinct group, found only in the 

 Older Palaeozoic rocks, and most abundantly preserved 

 in black shales. Their habit of growth is more sugges- 

 tive of plants than animals, consisting as they do of 

 numerous similar parts borne on a stem which may be 

 branched. Such a habit is, however, known in living 

 aquatic forms which are undoubtedly animals : it is 

 easily arrived at when very simple structure is associated 

 with a fixed habit. So far as this feature goes the 

 graptolites might be assigned to any one of several 

 animal phyla, but the balance of opinion is in favour of 

 regarding them as hydrozoa (see later, p. 296). The 

 names given to the parts of the graptolite are chosen 

 on this assumption. 



The skeleton of graptolites is composed of some 

 organic material. When preserved in shales it is usually 

 crushed flat, and appears as a thin film, sometimes of a 

 whitish material, more often of graphite. More rarely 

 the material was replaced by pyrite before being crushed, 

 and still more rarely it has been preserved uncrushed 

 and unaltered in limestone from which it can be ex- 

 tracted by treatment with acid. 



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