C. gemmatum tends to be fairly light in weight, rather delicate looking for
a Cymatium, with finer sculpture. C. mundum is "chunky" and noticeably more
solid, with wider, more rounded varices and generally coarser sculpture. C.
gemmatum has a greater length-width ratio, due to a taller spire and a
longer siphonal canal. C. mundum is usually white or off-white, while C.
gemmatum is usually colored - brown, orange, reddish, etc. frequently with
darker transverse bands.  These two are one of those "species pairs" which
are fairly easy to tell apart visually once you have seen several of each,
but which are difficult to write descriptions which will enable someone else
to identify a particular specimen - like Cypraea tigris and Cypraea
pantherina.