For the past 5 or 6 years, I have been photographing the exterior
portions of animals of molluscs.  Each genus or subgenus has a
characteristic layout of their basic parts.  All the animals in a group
would have similar animals in this respect.  Occasionally, you find a
species that does not match the others in the genus/subgenus and it
probably doesn't belong in the group.  Individual species usually have a
characteristic color pattern, though the individual spot sizes and
locations vary as a "finger print".
 
Close attention to the opercula will also follow in a group.  If the
growth pattern of opercula of otherwise similar shells differs
significantly, they are not likely to be the same species.
 
Within a short while of placing a mollusc in alcohol, the colors that I
see in photographs disappear.  Working on preserved animals only, a
person would not be able to appreciate collor patterns.
 
Working biologically on the animal, which I do not do but professionals
do, there are considerable differences between groups in things such as
types and placement of sexual organs, digestive systems, and nerve
systems which are a final determination of whether specimens are in the
same genus/subgenus.  Radula have been shown to differ between species,
though it is not very apparant to untrained observers.  DNA is very
useful in separating groups and species, but not many conchologists
would be able to use this information.
 
Aloha,
 
Wes
 
Many of these items are not pertinent to Pectinidae, but some are
pertinent in determining whether two specimens are the same species.