Scott,

They can be very similar but there are some basic differences that will
usually differentiate C. cymbium and C. cucumis.

C. cucumis                              C. cymbium

1. Overall shape tapers toward spire    1. Overall shape tapers only
so that shell looks like the tip has    slightly so shell looks like the
been cut off.                           top 1/2 had been cut off.

2. Grove between spire and the ridge    2. Grove between the spire and the
on top of the whorl is deep.            ridge on top of the whorl is almost
                                        completely filled in and flat.

3. Ridge on top of whorl around spire   3. Ridge on top of whorl around
is almost level from where it leaves    spire is not level but spirals
the spire to the end of the aperture.   towards siphonal canal end of
shell.

There may be (probably are) better ways to tell these two apart but I
think these three traits will do well in most cases.  Last, size for
size, C. cucumis always seem to feel heavier than C. cymbium.

Good luck,

Tom Eichhorst in New Mexico, USA

P.S.  If you would like I can scan my two specimens and send you the
image so you can see the differences (at least in my shells).