1092 



PRINCIPLES OF STRATIGRAPHY 



posit and the skeletons of rodents have been found in the expansion 

 at the base of the erect spiral. The material which has filled the 

 burrow has solidified and now forms a solid core or mold of the 

 original burrow. It should, however, be said that sections of the 

 core disclose what appears to be a cellular structure, which has led 

 to the supposition that the D^emonelix is not a burrow but a plant, 

 which grew around the skeleton and has been preserved in the atti- 

 tude of growth. 



The borings of sponges in shells (Clione) and the excavations 

 made by molluscs and echinoderms in wood and stone represent 



Fig. 261. Two views of a typical example of Dcruwnclix circiimaxilis, from 

 the Miocenic beds of Nebraska. (After Barbour.) 



permanent lodgments of the organisms, and are more nearly of 

 the grade of artificial structures than is the case with the burrows 

 before mentioned, which are more transient, and more nearly re- 

 lated to trails made in transit. For illustration of the burrows of 

 echinoids of limestone in Brazil, see Branner (8). 



Burrows like the Devil's Corkscrew, above described, if, indeed, 

 they are burrows, and like the borings of aquatic animals, are pre- 

 served by reason of the character of the material in which they 

 were excavated. Worm-tubes, on the other hand, so characteristic 

 of the sandy and muddy beaches, are maintained by a lining or 

 cement of mucus, secreted by the animal. These, therefore, 

 carry us a step further into the class of undoubtedly "artificial 

 structures." 



