ARTHROPHYCUS AND D^DALUS. 2O9 



volutions in nearly the same plane, cuts itself 9 times. Several of 

 these cuttings are represented in Figure 3 of its base. 



The self-penetrations by giving the relative ages of different parts 

 of the spirals, enable us to determine the direction of formation in 

 Disdains. Examination of specimens showing such cuttings estab- 

 lishes the cylinder as the terminal element and shows the direction to 

 be a general downward one. In the flat plates the descent is direct, 

 but in the flexuous and spiral forms it is more or less gradual or there 

 may be none. 



The cylinder is always present, but the number of component 

 strips varies with the length. If we should in imagination trace the 

 development of a plate back to its beginning, we should expect a con- 

 tinually lessening number of strips as it became shorter and shorter, 

 until at last only the cylinder would remain. Such isolated cylinders 

 are found. They are roughly J-shaped and at the lower end taper to 

 a blunt point. The corrugations are limited to the under half of the 

 surface. 



In Arthrophyciis, the direction of formation was just the reverse of 

 that of Da;dahis, and the last strip fitted to an overlying cylinder. 

 Isolated cylinders having a more open curvature than those referred 

 to Dcrdalus are sometimes associated with them. 



The clue to the nature of Arthrophycus and Dccdaliis is given by 

 the penetrations. They show that these objects were not organisms. 

 The structures or portions of structures cut were in all cases casts. 

 The spirals which penetrate themselves were already casts as far down 

 at least as the point of penetration, while still in process of formation. 

 Casts, of course, could exist only in the sediment. 



In my opinion the isolated cylinders are the casts of simple bur- 

 rows. The plates of Dczdalus were formed by repeated packings of 

 sediment in the upper side of the lower portion of a J-shaped burrow 

 while it was being shifted. Each of the curved lamellae represents a 

 packing. The cylinder represents the burrow in its final position (see 

 Diagram i). The packings were made largely in disposing of sedi- 

 ment sifted into the burrow by tidal currents and other agencies. 

 The direction which the shifting took adjusted the length of the 

 burrow to the growth of the occupant. The most rapid increase in 

 length was effected by the direct descent seen in the fiat plates. The 

 lateral shifting which generated the flexuous plates, and when uniform 

 in direction, the spirals, became less and less effective in lengthen- 



