IV.—Shell-Structure in the Ammonoidea

The shell-structure of the Ammonoidea has received very inadequate attention from palæontologists; in most cases authors are content with describing the shell of the recent Nautilus, and saying that that of the Ammonites is exactly the same. Prof. Judd therefore suggested that I should take the subject up, when working as a research student under him. As a result, I have found that the resemblance of the Nautilus- to the Ammonite-shell is literally true; but the structure of the extinct forms throws an altogether new light on that of the recent one, while the usual homology of parts between the two requires, I think, revision.