1) If a growing structure is built up of successive parts, similar in form magnified in geometrical progression, and similarly situated with respect to a center of similitude, we can always trace through corresponding points a series of equiangular spirals.
2) It is characteristic of the growth of a shell...that each successive increment of growth is similar, and similarly magnified, and similarly situated to its predecessor, and is in consequence a gnomon to the entire pre-existing structure.
3) Conversely, it follows that is the spiral outline of the shell we can always inscribe an endless variety of other gnomonic figures, having no necessary relation, save as a mathematical accident, to the nature or mode of development of the actual structure.