780 THE EQUIANGULAR SPIRAL [ch. 



other words, we can imagine our shell cut up into a system of rings, 

 following one another in continuous spiral succession, from that 

 terminal and largest one which constitutes the hp of the orifice of 

 the shell. Or on the other hand, we may figure to ourselves the 

 whole shell as made up of an ensemble of spiral lines in space, each 

 spiral having been traced out by the gradual growth and revolution 

 of a radius vector from the pole to a given point on the boundary 

 of the generating curve. 



I 2 



Fig. 374. 1, Harpa; 2, Dolium. The ridges on the shell correspond 

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



Both systems of fines, the generating spirals (as these latter may 

 be called)! and the closed generating curves corresponding to suc- 

 cessive margins or hps of the shell, may be easily traced in a great 

 variety of cases. Thus, for example, in Dolium, Eburna, and a 

 host of others, the generating spirals are beautifully 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 



