Note 77 — VIII.

The periodical formation of these septa in the progress of growth, is analogous to that of the projecting external plates in the Wendletrap, and of the rows of spines in the Murex; but those external processes consist of the opake calcareous layer of the shell, whilst the internal processes in the Nautilus consist of the nacreous layer, like the septa in the Turritella. Thus the embryo Nautilus at first inhabits a simple shell, like that of most univalve Mollusca, and manifests, according to the usual law, the general type at the early stage of its existence; although it soon begins, and apparently before having quitted the ovum, to take on the special form.—Prof. Owen's Lect. on Invertebrate Anim. p. 593, 2d Ed.