NATURE OF TAONURUS. 



213 



or oval plates, beginning at one side they cross with a curvature 

 nearly the same as that of the base and ascend along the other. The 

 edge of the plate is formed by a narrow border, usually flat, but 

 sometimes in better preserved specimens, distinctly cylindric. In the 

 spirals the ridges radiate outward from the apex in sickle-shaped 

 curves to meet the margin at a low angle, recalling the lines of sparks 

 given off from a pin-wheel. In many spirals, however, at the upper 

 end of the top volution, they are arranged in such a way as to sug- 

 gest the form and appearance of one of the U-shaped plates. The 

 free edge of the spiral is formed by the marginal cylinder, which at 

 the base of the structure rounds in and upward and passes through 

 the center. In some well-weathered specimens preserved in sand- 

 stone, the curved ridges are seen to be the edges of strips which are 



Fig. 2. — Spiral form ol Taonuriis ; t", cylinder. 



lunate in section and saddle one upon the other, the last of the series 

 partially enclosing the cylinder. 



Frequently these structures are found passing through others, or 

 one part of a spiral may pass through another part. These penetra- 

 tions could not have occurred had these structures been organic. 

 The self-penetrations are always the cutting of an upper volution by a 

 lower. This indicates downward formation. It also shows that these 

 structures originated in the sediment, for the portions cut were casts, 

 and as such could exist only in the sediment. 



As intimated above the key to the nature of Taonunis was given 

 by a study of Dcedalus. That genus comprises simple J-shaped 

 cylinders and U-shaped, irregularly flexed, and spiral plates, much 

 thicker than those of Taonurus (see Figures i and 2, pages 204, 205 

 of this volume.) The U-shaped plates stand vertically in the rock 

 instead of reclining, and the volutions of the spirals generally form 

 acute cones, instead of depressed, and are practically uniform in size in 



