84 INVERTEBRATE PALAEONTOLOGY 



against permanence of preservation. The process of 

 filtration that converts alluvium into gravel involves ex- 

 posure of shells to the devastating action of percolating 

 water (by which even Mammoths' tusks are destroyed) 

 while an ordinary gravel terrace assumes in time a 

 position so exposed that its removal by denudation is 

 well-nigh certain. The remains of fluviatile organisms 

 can only attain lasting preservation when drifted into 

 a lake or sea, and the vicissitudes of their journey are 

 consistently destructive. 



Swamp- or lake-deposits usually possess some virtue of 

 constancy, and normally accumulate under very tranquil 

 conditions. They are often argillaceous, and sometimes 

 peaty, affording fairly safe burial to calcareous shells. 

 In the case of aquatic Molluscs of high antiquity, the 

 same uncertainty as to adaptive structure exists as in 

 terrestrial forms ; it is more from evidence of their 

 associates than from any intrinsic qualities that 

 Archanodon from the Old Red Sandstone or Carbonicola 

 from the Coal Measures are regarded as fresh-water 

 mussels. The Unionidae of the Yorkshire Jurassic, the 

 Cyrenidae of the Wealden, and the Limnaeidae of the 

 Oligocene give more certain indications of the lacustrine 

 or deltaic conditions of their surroundings, since they 

 clearly represent groups whose present distribution is of 

 that nature. Shell-marls, often interbedded with peat, 

 are frequent in the later Tertiary deposits, and in most 

 cases mark lacustrine phases of sedimentation. 



The Arthropods inhabiting fresh-water are usually 

 small, and belong, for the most part, to groups equally, 

 or better, represented in marine faunas. There is little 

 difference in the shells or carapaces of members of 

 allied groups inhabiting the two provinces. Among 

 Ostracoda, the Cypridae usually prefer fresh water ; their 

 remains are particularly abundant in the deltaic parts of 



