514 



THE LOGARITHMIC SPIRAL 



[CH. 



mathematical accident, to the nature or mode of development 

 of the actual structure*. 



Fig. 258. A shell of Haliotis, with two of the many lines of growth, or generating 

 curves, marked out in black : the areas bounded by these hnes of growth being 

 in all cases "gnomons" to the pre-existing shell. 



Fig. 259. A spiral foraminifer {Pulvinv Una), to show how each successive chamber 

 continues the symmetry of, or constitutes a gnomon to, the rest of the structure. 



* For many beautiful geometrical constructions based on the molluscan shell, 

 see Colman, S. and Coan, C. A., Nature's Harmonic Unify (ch. ix, Conchology), 

 New York, 1912. 



