>Thanks Andrew - as usual you are a font of knowledge!!!  My specimen is
>about 70mm, and has a VERY open umbilicus of several mm - 5 or 6, with
>no indication that it is in the process of closing.   Perhaps this in
>itself is unusual - i recall another similar-sized specimen that had a
>completely closed umbilicus, just like in a normal adult.

Some Australian specimens can lack the umbilical plug (right, left or
both) right into adulthood; these have been called N. stenomphalus, a
synonym of N. pompilius. The umbilicus can be relatively "wide",
though of course far narrower than N. macromphalus or A.
scrobiculatus (Allonautilus I think MAY be a synonym of Cenoceras, a
Triassic-Jurassic genus of Nautilidae, and direct ancestor of
Nautilus as well).
--
Regards
Andrew