I touched on the idea of waves smashing shells coming into beaches. Water motion also contributes to specific wear on some shell species.

Large, heavy Busycon such as Busycon carica and Busycon sinistrum frequently have a worn spot on the previous whorl adjacent to the aperture just outside the columella. Large examples of the species tend to role over with the aperture facing down. Wave action and currents shifting the shells back and forth or along the bottom presumably is the cause for the thinning and ultimately wearing through of the shell whorl. It is a process that begins quickly upon death of the animal thus finding an example without anyt of the wear is more the exception. I see the same wear to a less frequent extend on Busycotypus canaliculatus but have never observed it on Busycotypus spiratus, both much lighter and thinner species - even at full adulthood - which have a more evenly distrubuted center of gravity thus more likely to role than slide when moved by water.   

Another water action wear occurs on certain bivalves with a tough hinge ligament. One is Dinocardium robustum. As the shell tumbles around the valves move back and forth producing a characteristic wear on each side of the hinge to the facing valves. This is not to be confused with drilling by moon snail or murex. It is most evident when finding both valves still attached cast up on the beach. The wear is flat as though the area was simply filed off rather than bowl shaped as a predator would make. It is also evident equally on each valve rather than the one one valve that a predator typically would leave.  

John Timmerman
Wilmington, North Carolina

USA

-------------- Original message from David Kirsh <[log in to unmask]>: --------------


> Does anyone know whether there has been much study of the common ways
> that shells are eroded or broken?
>
> I've noticed that I can be walking on an Atlantic beach with someone
> who doesn't seriously collect and they'll pick up a (to them)
> attractive fragment and I'll know from a small sliver what it is. (But
> I might not be able to explain how I know). How many ways are there for
> Neverita duplicata to be eroded and broken and sliced by waves and
> other environmental factors? It seems like there are quite a few
> "forms." It can be challenging to identify certain atypical fragments
> or eroded shells of less familiar species.
>
> Some shells are known mainly in their incomplete state. For in stance,
> they might be invariably "decapitated" by the time they're adult. In
> such cases, we're unfamiliar with what they would look like intact.
> Just yesterday, I found a young Truncatella with its protoconch intact
> and I'm hoping to post an image of that soon.
>
> David Kirsh
> Durham, NC
>
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