there originates a system of U's in a U. Between the limbs of the outer U a 

 "Spreite" is formed which can be compared with the web of a duck's foot. 

 Beginning in the Paleozoic we know such U-shaped burrows with "Spreite", 

 also in much more complicated shapes. In recent biotopes they are also known 

 from other, non -marine regions and fossil occurrences do not permit a conclus- 

 ion of fossil tidal flat regions, not even of marine environment. 



Among the arthropods the little amphipod Corophium leaves a variety 

 of trails and burrows. Thus, in fossil state one would not ascribe them to a 

 single animal. It makes two different forms of crawling trails, a smooth one 

 and one resembling a string of pearls; it digs a little U-shaped burrow, either 

 perpendicularly into the surface or horizontally into the walls of the tidal chan- 

 nels. While feeding it produces a little, rather regular star-like feeding trail. 

 The form of the burrow is not always a U. In sandy sediments, which are better 

 ventilated than muds, the animal often digs a simple vertical shaft. 



The well-known Crangon offers a good example of that which Rudolf 

 Richter termed a "resting trail" (or Kuenen reposing trail, in German "Ruhe- 

 Spur"). This arthropod is to be found hidden in small pools in the surface of 

 the sand during ebb tide. It leaves there flat, uncharacteristic and usually 

 transient troughs. Commonly they are oriented parallel to one another, be- 

 cause the animals faced the current. This parallelism is a characteristic 

 feature for this kind of trail which is also to be found in fossil resting -trails 

 produced by some kinds of arthropods. Many different arthropods in the 

 ancient seas have made various kinds of burrows, often rather characteristic- 

 ally branched or expanded. On. our tidal flats we know of no recent examples 

 for such burrows as the screw-like trace -fossil Xenohelix from the Tertiary 

 of North America. Only Carcinus can be seen, retired in burrows ending with 

 a hollow which have been dug in the walls of tidal channels. 



Among the more sessile pelecypods only few traces of their activity are 

 recognizable. Cardium seldom crawls and its trails are only short furrows on 

 the surface of the sand. Star-like feeding trails can be observed which are 

 made by Scrobicularia. This pelecypod, living about 5 inches below the surface 

 of the tidal flats in sandy sediment, extends one of the long and separated siphons 

 (the incurrent siphon) to the surface of the sediment. It produces rather regular 

 stars when it draws in sediment from the surface in various directions. Such 

 trails can be named (according to A. Seilacher) feeding or grazing trails, in 

 German "Weide-Spuren". The siphons of Mya are joined together and they pro- 

 duce rather large holes by which this pelecypod can be detected. 



Among the gastropods Littorina is often to be seen wandering over tidal 

 flats, especially near high water line. Sometimes the trails are parallel for 

 long distances and not seldom one animal creeps behind the other in the same 

 trail. If one of them leaves the furrow, the trail becomes branched which is 

 somewhat unexpected in gastropod trails. The gastropod Hydrobia is very a- 

 bundant on the tidal flats but produces trails of no special interest. These are 

 simple, smooth furrows which are irregularly curved and which resemble those 

 produced by other little animals such as crustaceans. 



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