The family Nautilidae, of subclass Nautiloidea, contains the following
living taxa:
genus ALLONAUTILUS Ward & Saunders 1997
-A. scrobiculatus (Lightfoot 1786)       New Guinea, Solomon Islands
-A. perforatus (Conrad 1847)             Bali
genus NAUTILUS Linne 1758
-N. belauensis Saunders 1981             Belau Republic (Palau, Micronesia)
-N. macromphalus Sowerby 1848            New Caledonia
-N. pompilius Linne 1758                 IndoWestPacific (=repertus)
-N. stenomphalus Sowerby 1849            NE Australia

N. belauensis shells differ from N. pompilius in that the former is of
larger adult size and has a distinct ornament of spiral lirae and collabral
(parallelling the growthlines) sculpture. The strong sculpture of N.
belauensis, as in A. scrobiculatus, supports a periostracum thicker than
that of           N. pompilius.

In N. pompilius the spiral lirae are almost nonexistent and the collabral
sculpture is absent. Radulae differ also.

N. stenomphalus differs from the previous two in that its very narrow
umbilicus is not plugged by callus, and it is possible to see through the
perforate umbilicus to the other side (generally the case in nautiloids
with open umbilicus). Its shell is smooth and has much less red pigmented
area than any other species, and the animal differs visibly.

N.macromphalus has a larger amount of red pigmented area than the others, a
broader shell and has much wider umbilicus than N. stenomphalus. Its shell
is also essentially smooth.

The "paper nautilus" Argonauta is an octopod, as stated by Paul Monfils,
producing an extremely thin and fragile eggcase, not a proper shell at all,
which is formed by a pair of modified arms in the female. It is not
attached to the animal. While it appears to be coiled, I believe the whole
thing is formed at one go (I don't have a specimen, so cannot check for
growthlines to test this idea).

"Ammonites" of popular terminology belong to subclass Ammonoidea, and
should strictly be referred to as ammonoids. Ammonoidea is divided into
orders Bactritida, Goniatitida, Clymeniida, Ceratitida, Ammonitida. Only
the latter should be referred to as ammonites. They are derived from
nautiloids, ie the highly conservative nautiloids were ancestors of the
more "progressive" ammonoids. Typically (but not necessarily) ammonoids
have a much thinner shell than nautiloids, with septal walls which (in
order of the orders listed earlier in this paragraph) with time became
increasingly more complexly folded where they join the shell wall. It is
becoming clear that ammonoid shells were far more complex than those of
nautiloids, with various membraneous structures etc within the camerae. The
animals are unknown, but it increasingly seems that they were
planton-feeders, possibly with very fine netlike arms. They were probably
more coleoid, ie more similar to octopuses and squids, than to today's
nautiloids. Earlier nautiloids were also more squidlike, as we know from
the rare preserved animals. Some had inksacs!

Both ammonoids and nautiloids add new chambers (correctly called camerae;
singular camera, meaning "room") periodically to their shells; at first
these new chambers are filled by a fluid that is not quite seawater, and
are slowly emptied by osmosis via the siphuncle, an organic structure
running through septal necks perforating the septa and connecting all the
chambers. The next new chamber begins forming before the previous one is
completely empty. The cameral gas which replaces the fluid is similar to
but of slightly different composition to air, and is never at more than one
atmosphere pressure, irrespective of depth. Once emptied, a chamber will
not refill with fluid unless there is a structural failure of the shell or
siphuncle. That is, the animal's osmotic pump cannot be reversed, and
cannot pump liquids into the chambers. The animal's ability to migrate
vertically in their diurnal cycle is a function of active swimming, not of
buoyancy.

A good reference for Nautilus is Peter D Ward's 1987 "The Natural History
of Nautilus", Allen & Unwin, ISBN 0-04-500036-0.

Meaningful information about ammonoids generally is restricted to
scientific literature. Research on ammonoids and their ecology is at a very
early stage and we really know very little about them as yet. The Treatise
on Invertebrate Paleontology is old and utterly outdated, though being
rewritten; the old ammonoid volume L is being replaced by several volumes,
only one published so far. In order to get some idea of what ammonoids were
like it is necessary to read some numbers of very recent papers by various
authorities; to read only a few will give a biased and highly incomplete
view. Some authors of very recent work worth following up are: Bruce
Saunders, Peter D Ward, Gerd E G Westermann and Adolf Seilacher. Seilacher
in particular is full of interesting ideas.

I hope this is of interest.

Andrew