16 INVERTEBRATE PALAEONTOLOGY 



much of the Wealden and the Reading clays, need not 

 be ascribed to a paucity of fauna ; it is more probably 

 due to the causes mentioned above. 



The sea is the chief receptacle for the products of 

 rock-wastage, and it is a fortunate coincidence that the 

 majority of shell-bearing Invertebrates (excluding the 

 Insects) are marine. Deposits accumulate in greatest 

 bulk, and most rapidly, in the "detrital zone," which 

 extends over the usually shallow parts of the sea around 

 coast-lines. Again the satisfactory phenomenon can be 

 recorded that the richest zones of Invertebrate marine 

 life (perhaps excluding the Protozoa) coincide very 

 nearly with the belt of coastal water where prompt 

 burial is most likely to follow death. Further, rocks of 

 the detrital zone are the most favourably situated for 

 subsequent elevation, so that the fullest records of past 

 life available for palaeontological study are normally 

 those of the denizens of the marginal tracts of the sea. 

 Organisms left upon, or drifted to, the beach will almost 

 certainly suffer from the abrading influence of waves, 

 and may become exposed to atmospheric weathering. 

 The larger Invertebrates of deep water are, for the most 

 part, inefficiently clad in mineral armour, and hence are 

 not well adapted for preservation intact. The ideal 

 situation for fossilization is in the shallow water below 

 the level of low tides, where the scour of currents 

 and waves is insufficient to prevent deposition of fine 

 silt. Corals, Sea-Urchins, Brachiopods, Molluscs and 

 Crustaceans are the groups that abound, or have 

 flourished, most in such surroundings ; their remains 

 constitute the chief part of the palaeontological evidence 

 available for study of the larger Invertebrates. 



