XI] OF THE MOLLUSCAN SHELL 779 



us call this closed curve the "generating curve"; the surface which 

 it bounds we may call (if need arise) the "generating front," and 

 let us imagine some one characteristic point within this closed 

 curve, such as its centre of gravity. Then, starting from a fixed 

 origin, let this characteristic point describe an equiangular spiral 



Fig. |373. Melo ethiopicus L. 



in space about a fixed axis or "conductrix" (namely the axis of the 

 shell), while at the same time the generating curve grows with each 

 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. 



The resulting shell may now be looked upon in either of two ways. 

 It is, on the one hand, an ensemble gf similar closed curves, spirally 

 arranged in space, and gradually increasing in dimensions in pro- 

 portion to the increase of their vector-angle from the pole*. In 



* The plumber, the copper-smith and the glass-blower are at pains to conserve 

 in every part of their tubular constructions, however these branch or bend, the 

 constant form which their cross-sections ought to have. Throughout the spiral 

 twisting of the shell, throughout the windings and branchings of the blood-vessels, 

 the same uniformity is maintained. 



