I thought this was a nautilus, but it might be a moon snail. It’s a mollusk of some kind, with a gracefully curved shell and a complex, fleshy body, all rendered as simply as possible in pen and ink. On the page, it’s a sort of textual decoration more than a scientific illustration. The monograph (which I found here in the Biodiversity Heritage Library) lists a few octopuses and squids, but nothing that comes in its own chambered shell – and none that are illustrated. No, the only picture is this little guy at the bottom of the last page, standing as a kind of cipher for all the tentacled mollusks in their many-limbed glory, and maybe the humble yet beautiful gastropods as well. A lesser publisher would have put a flower or some kind of curlicue to decorate this spot.