526 



THE LOGARITHMIC SPIRAL 



[CH. 



traced out by the gradual growth and revolution of a radius 

 vector from the pole to a given point of the generating curve. 



Both systems of lines, the generating spirals (as these latter 

 may be called), and the closed generating curves corresponding 

 to successive margins or lips of the shell, may be easily traced 

 in a great variety of cases. Thus, for example, in Dolium, 

 Eburnea, and a host of others, the generating spirals are beautifully 



Fig. 266. 1, Harpa; 2, Dolium. The ridges on the shell correspond 

 in (1) to generating curves, in (2) to generating spirals. 



marked out by ridges, tubercles or bands of colour. In Trophon, 

 Scalaria, and (among countless others) in the Ammonites, it is 

 the successive generating curves which more conspicuously leave 

 their impress on the shell. And in not a few cases, as in 

 Harpa, Dolium perdix, etc., both alike are conspicuous, ridges 

 and colour-bands intersecting one another in a beautiful isogonal 

 system. 



