XI] 



OF THE MOLLUSCAN SHELL 



525 



origin, let this centre of gravity describe an equiangular spiral in 

 space, about a fixed axis (namely the axis of the shell), while at 

 the same time the generating curve grows, with each angular 

 increment of rotation, in such a way as to preserve the symmetry 

 of the entire figure, with or without a simultaneous movement 

 of translation along the axis. 



It is plain that the entire resulting shell may now be looked 

 upon in either of two ways. It is, on the one hand, an ensemble 



Fig. 1^5. Melo ethiopicue, L. 



of similar closed curves spirally arranged in space, gradually in- 

 creasing in dimensions, in proportion to the increase of their 

 vectorial angle from the pole. In 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 lip 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 



