GEOLOGICAL PALAEONTOLOGY 8; 



flourishing in muddy or sandy water. The fauna of the 

 open sea can be divided in accordance with depth into 

 planktonic, or floating, and benthic, or abysmal, types. 

 Fossil evidence is hardly needed for recognition of the 

 state of purity of sea-water, since the lithology of 

 the sediments is a sufficient guide. As a general 

 rule, Sponges, Corals, Echinoclerms (other than Heart- 

 Urchins), Polyzoa and Brachiopods avoid muddy 

 surroundings, while Molluscs and many kinds of 

 Arthropods find food and happiness where the water is 

 thick with falling silt. Cessation, or extreme slowness, 

 of deposition often gives opportunity for the growth of 

 boring or incrusting organisms ; such forms frequently 

 indicate non-sequence in apparently continuous sedi- 

 ments. Tubular perforations due to Pholas, and clusters 

 of Ostreidae, indicate that the layers affected had 

 attained a fair degree of hardness before overlying 

 sediments were laid down. 



Evidence of the depth at which deposits were formed 

 is made uncertain by two independent conditions. 

 Provided that their remains can resist solution, plank- 

 tonic organisms may sink to the floors of abysses ; so 

 that more or less sedentary forms can alone be relied 

 upon for determination of depth. There can be little 

 doubt, however, that the restriction of an existing group 

 to any particular vertical zone in the sea is no criterion 

 of its past habitat. Most of the recent stalked Crinoids, 

 and a large proportion of Brachiopods, occur in very 

 deep water ; but in the past, when these groups were in 

 their prime, their distribution was far more varied, and 

 even brought them within the tidal rone. There is a 

 marked tendency for members of a waning class to 

 retire to the relative quietude of the deep sea ; but this 

 tendency is not a definite rule, so that discovery of 

 senescent types cannot, of itself, give proof of benthic 



