Hi Peter,
Where are you located?  Sounds like north of Cape Cod, judging from the
species you mentioned.  I'm in Rhode Island.  We have all the same species you
mentioned, however Lunatia heros (northern moon snail) is only found in deeper
water here - they need cold conditions.
Scars on gastropod shells fall generally into two categories - growth lines
and repaired injuries.  A growth line occurs when the growth of the animal
(including its shell, of course) slows down dramatically, or even stops for a
while, and then resumes.  A growth line normally runs the full length of the
body whorl, and is parallel to the lip.  It can range from a fine, almost
invisible ridge, to a deep groove in the shell surface.  An interruption in
growth can be caused by various factors such as a temporary food shortage or
non-lethal disease, but the commonest cause is temperature fluctuation.
Mollusks, like other invertebrates and cold-blooded animals, become more
active, and metabolize and grow faster as the temperature gets higher.  The
Ilyanassa obsoleta you mentioned are an interesting case, all having a growth
line the same distance from the lip.  I would guess that they have recently
resumed growing, after a virtual lack of growth during the frigid winter
months.  Being members of the same geographic population, they would be
subject to similar temperature and nutrient conditions, so they would resume
growing at about the same time, and at about the same rate; hence, a similar
production of new shell growth beyond the line that marked the period of
inactivity.  You say the growth line is about 5-6 mm above the lip.  Check the
population in a couple of weeks, and see if the growth lines are then more
than 5-6 mm above the lip.  In fact, if you measure the lip-to-line distance
on a fairly large number of specimens, and take an average - and then do the
same thing on a second visit - you can calculate the average linear amount of
new shell the snails are producing per week, or per day.
The other, usually more irregular scars that are found on gastropod shells are
the result of the lip being chipped or broken at some time in the animal's
development.  The animal then continues to grow, and the chip or break is
filled in with new shell material.  However, the junction of the new material
with the broken edge of the old material is usually not very well blended, and
a visible scar results.  Such scars are often referred to by collectors and
dealers as "reef breaks" or "growth mends".  They can be straight, curved, or
jagged, and can be oriented parallel, oblique, or even perpendicular to the
lip.  Usually they do not run the full length of the body whorl, though that
is possible.  These can range from a smooth ridge, bump, or dimple which is
not supposed to be there, to a jagged, rough, ugly ridge that looks just like
what it actually is - two pieces of shell that were displaced and then poorly
cemented together.  Such chips and breaks can be due to attacks by predators
like fish, crabs, lobsters, etc.;  by human activity, especially the tackle of
commercial trawlers and draggers;  and, in shallow water, by the action of
waves, especially storm waves, tossing the shells around among the rocks.
Incidentally, such growth lines and repairs are not limited to gastropods.
Everything I said above also pertains to bivalves; and mends are common on
both tusk shells and cephalopod shells.  I'm not sure how often chitons are
affected - they probably don't get damaged as often as other mollusks, due to
their way of life and the protection of their tough girdle.  But maybe some
chiton person can provide some information on that.
 
Paul M.