48 INVERTEBRATE PALAEONTOLOGY 



of lining shells in symbiotic relationship with their 

 owners, but after death similar animals may mask the 

 superficial features, external and internal, and penetrate 

 the substance; just as Barnacles encrust, and ship- 

 worms perforate, a wooden hull. Quite long " lists of 

 fossils" may sometimes be compiled by enumeration 

 of the species of Hydrozoa, Madreporaria, Polyzoa, 

 Brachiopoda, Serpulids and Pelecypoda adhering to 

 Chalk Echinoids, while the test itself may be so 

 smothered under these growths that even generic deter- 

 mination is doubtful. 



(V) IMPRESSIONS AND TRACKS 



The bodies of dead animals lying on soft silt, or those 

 of living ones moving across it, leave impressions more 

 or less corresponding with their shape and structure. 

 Thus the surface of a modern beach is broken by worm- 

 castings and tracks, burrows of Heart Urchins, Razor- 

 shells and jSand -fleas, footprints of Crabs, and stranded 

 forms of comatose Jelly-fish. In most cases these traces 

 of abundant life are obliterated at the next tide; but 

 since, like ripple-marks, they extend into shallow water 

 below tidal limits, they may be covered by new layers 

 of sand or clay without much disturbance if the weather 

 is calm and deposition is in progress. Just as ripple- 

 marks, and other inorganic irregularities, are commonly 

 found in littoral deposits of all ages, so organic vestiges 

 are often perpetuated. The perfection of such markings 

 depends largely on the texture of the silt, while their 

 interpretation is bound to be uncertain if their makers 

 belonged to extinct groups. The sinuous grooves and 

 coiled casts of " lug-worms," and the blurred tracks of 

 Nereids, look much the same whether made in Lower 

 Palaeozoic times or yesterday. The confused, but re- 



