2IO ROCHESTER ACADEMY OF SCIENCE. 



ing the burrow as the rate of descent diminished. For 

 example, in one specimen collected, in which there are between 8 and 

 9 volutions and probably in the neighborhood of 800 packings, the 

 descent is only 3}^ inches. The directness of descent was undoubtedly 

 affected to some degree by the conditions at the surface. If denuda- 

 tion was going on, the destruction of the upper portion of the burrow 

 would need to be offset ; if on the other hand, sediment was accumu- 

 lating over the spot, the aperture would be raised and the rate of 

 descent would be retarded according to the rapidity of deposition. 

 Variations in the factors governing the direction of shifting resulted in 

 the formation of plates which are flat in one portion and flexuous or 

 spiral in another. Deviations in the direction often led to the bur- 

 row's cutting through some portion of the earlier packings which it 

 could do just as it could through the surrounding sediment or a neigh- 

 boring cast. 



In Arthf'ophycus the insifting sediment, instead of being packed 

 against the upper side of the lower portion of the burrow, appears to 

 have been distributed along the entire under side, thus producing a 

 progressive shifting of the whole burrow (see Figure 4). 



With each shifting the lower end was extended, and this tended to 

 keep the burrow at about the same depth and inclination; it maintained 

 the length or increased it, the amount needed, as in De^dahts, depend- 

 ing upon the amount of sedimentation. In many cases the animal is 

 seen to have shifted laterally, first one way then tJie other. From the 

 flabellations it appears that it sometimes drew out from a burrow and 

 made a new one beside it; for usually the cuttings show that the 

 different series of packings comprising a flabellation were formed suc- 

 cessively from one side of the group to the other. 



The packings in Arthropliyciis and Dcedalus were probably made 

 by pressure exerted by the animal's body. From their distinctness it 

 seems likely that they were separated by some secretion, like a film of 

 tenacious, quick-hardening mucus, added to make the material packed 

 more cohesive. By the same pressure by which the creature com- 

 pacted the insifting material on one side, it crowded itself into the 

 sediment on the other. When this was silt, it took an impression of 

 the minutest details of the body. 



The animals which formed these burrows were probably sedentary 

 Polychaeta. 



