INVEETEBEATE ANIMALS IN PALEOZOIC EOCKS. 611 



. Photograph No. xi. (not figured) shows another combination, 

 ■where numerous trails formed in soft sediment have been affected 

 by shrinkage-cracks, produced by the drying of the mass, in such a 

 manner as to present a very complicated appearance. 



Pig. 14. — Combination of Worm-trails and Ripple-marTcs. 

 Potsdam Sandstone ; Canada. ^ nat. size. (From a Photograph.) 



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Still another appearance which may be placed under this head is 

 that in fig. 15, which represents part of the surface of a large slab 

 of Calciferous Sandstone from St. Anne's. At first sight it seems 

 to be covered with a network of shrinkage-cracks, but on closer 

 inspection these are seen to be cylindrical worm-tracks or burrows 

 planed off and flattened on one side, as if a slab covered with casts 

 of worm-tracks had been rubbed or ground down till the originally 

 rounded sides of all the more prominent were flattened. The 

 only way in. which it seems possible to account for such an ap- 

 pearance is to suppose that the tracks were partly filled with mud 

 incapable of hardening into stone, and then completely filled and 



