I liked Wes Thorsson's notes on rarity. I'd just like to point out that
virtually any species is "rare" at the edges of its distribution. There,
even the collection of a few specimens could conceivably damage the local
population. The more collectors know about shells before they collect
(including their natural distribution), the better.
 
As to the word "rare", in botanical Latin it meant "sparse", hence
"sparsely distributed", hence "uncommon". For example, a tree with widely
spaced branches might be said to have "rare" branches. We have some rare
birds in Conch-L.
 
Andrew K. Rindsberg
Geological Survey of Alabama