<Can  you help me distinguish between:
(a) Belauensis
(b) Pompilius
(c)"the rare paper nautilus"
(d) the ammonite (an ancestor?)>

Hi Ellen,
Welcome aboard the good ship Conch-L!

I'll comment on (b) and (c) because I never heard of Belauensis, and I'm sure
some of our fossil folks can provide some information on ammonites.

Nautilus pompilius is the commonest of the half dozen or so living species of
Nautilus, which are the only living cephalopods having a true external shell.
 Others include N. macromphalus from New Caledonia; N. repertus from
Indonesia through the Philippines; N. scrobiculatus from various South
Pacific localities; and N. stenomphalus, usually from the Philippines.  The
chambers are filled largely with gas.  The septae, or walls, separating the
chambers each have a central hole, through which passes a continuous slender
tube of living tissue, the siphuncle.  This organ regulates the amount of gas
in the chambers, and allows the animal to adjust its buoyancy, rising or
sinking in the water without any swimming motion.

The "Paper Nautiluses" (or Paper Nautili, if you prefer) are a whole
different story.  There are several species of these, in the genus Argonauta,
including A. argo (probably the one you are referring to); A. boettgeri; A.
hians; A. nodosa; and A. nouryi, all from the south to central Pacific.
These "shells" are extremely thin and fragile, and have the same general
shape as Nautilus shells, but are not chambered.  The animal is not actually
attached to the "shell" as a Nautilus, and most molluscs are, but is free
swimming like an octopus.  This "shell" is produced by the female prior to
laying eggs, and serves mainly as a brood chamber for the eggs.

Regards,
Paul Monfils