Interesting "clam" - would like an ID

[GinaD]

In all the tanks I've seen, and of all the invertebrates I've encountered, I have never seen one of these before - at least not this long, anyway.

The shell is extremely delecate - I've bumped into it several times over the past year, breaking pieces off the end, and I'm thinking if I didn't, it would be much longer than it is.
Right now, it's almost 3" long.  I discovered it about 2 years ago and it was really tiny, barely noticable, like 1/4" in dia. 

Any ideas what it might be?

Top-Down:


From the left side:


From the right side:

[Aquadiva]

almost looks like a razor clam.

http://www.divebums.com/week/2009/May25-2009/razor-clam_steve-gardner.jpg
A hitchhiker, I assume??

[GinaD]

Thanks for the pic - It's similar, but the shell is not nearly as smooth or thick.  Also it looks like the razor clam shells are pretty sturdy.  The shell on mine is extremely brittle - pieces broke off when it was lightly bumped.  The biggest difference that I can see, after doing a search on razor clams is that they have the hinge on the side.  Mine has it's hinge on the end that's attached to the rock.

Yes, it came in on the rock it's currently attached to as a hitchhiker.