case one part appears to us as the continuation of the tube, and the other part, merging with it by inôÙdisôÙtinôÙguishôÙable boundaries, appears to us as the septum36.
The various forms assumed by the septa in spiral shells37 present us with a number of problems of great beauty, simple in their essence, but whose full inôÙvesôÙtiôÙgaôÙtion would soon lead us into mathematics of a very high order.
We do not know in great detail how these septa are laid down; but the essential facts are clear38. The septum begins as a very thin cuticular membrane (composed apparently of a substance called conchyolin), which is secreted by the skin, or mantle-surface, of the animal; and upon this membrane nacreous matter is gradually laid down on the mantle-side (that is to say between the animalãs body and the cuticular membrane which has been thrown off from it), so that the membrane remains as a thin pellicle over the hinder