The shells of Cephalopods form an evolutionary series, traceable to an ancestor unknown, but nearly related to not yet coiled forms of the Cambrian. In all descendants from this common stock homologous parts are discerned. Hence facts regarding the growth of any one form may be applied to all. Of Cephalopods with chambered shells only Nautilus, Spirula, and Sepia exist. Spirula is so rare and its shell so retrograde that no theory has yet been built on it. Examination of Nautilus gave rise to the Secretion-hypothesis, shortly stated as follows:—As the animal grows, the adherent muscles and the cincture gradually advance, the siphuncle lengthening in proportion; thus a cavity is formed between the last septum and the surface of the visceral hump: this latter then deposits calcareous matter, beginning at the sides of the shell and proceeding towards the siphuncle round which it is continued backward. During the advance of the animal the anterior portion of the mantle secretes calcareous matter, which it deposits in successive layers on the margin of the aperture (Edwards and Wood).