Hi Linda,
There are certain pairs of species that I am confident I can tell apart at a
glance, yet when it comes right down to listing reliable criteria for
separating them, it isn't easy.  Murex superbus and Murex venustulus are one
such pair.  In looking through several books, I found little in the
descriptions that could be used to reliably separate these two.  The
descriptions "sound" almost alike.  I have about 20 superbus and about a
dozen venustulus on hand, so I lined them up and took a good look.
Characteristics like a taller spire and longer spines on superbus are useful
when you have the two species side by side, but not very useful when you are
trying to identify a single shell.  The best I can do is that venustulus
seems to have one major axial rib, halfway between the varices, and that rib
has a single large swelling centrally.  In superbus there are two parallel,
evenly spaced large axial ribs between the varices, and they are about the
same height along their full length, with no enlarged central swelling.
Also, while I agree that color is, generally speaking, not a good criterion
for separating species, the fact is M. superbus is usually quite constant in
coloration, a light ground color, off-white to light tan, with many
transverse dashes of darker color.  M. venustulus is very variable in color,
with solid reds, oranges, and yellows, plus color-banded forms.  If your
specimen is uniform peach color, without darker markings, I'd lean toward
venustulus, but of course that is not absolute.  Finally, superbus is a
larger shell, up to 80 mm commonly, and occasionally up to 100 mm.  M.
venustulus is usually 50 to 60 mm or so.  But of course, superbus are also 50
to 60 mm at some point in their growth.  Good luck.
Regards,
Paul M.