An Odd Bivalve Shell; Symbiotic Worms, or Killers?

It's ~3 3/4" long, ~3" wide, ~1 3/4" high; it weighs ~141 grams.
The area above the hinge, corresponding to the oldest growth-layer, seems to be the most riddled...but other bivalves I've seen usually show worm-penetration through into the living chamber, a sign of predation. This has not a single such penetration.
I'm a west-coast kid (US), an' not deeply informed even about our indigenous clams, mussels & etc., but the layered growth, an' the presence of worms that haven't fed on the critter within, are new in my experience. I'm betting this is from a distant sea-shore, an' hope for some of y'alls knowing eyes to tell me more about it. Thanks for lookin'!

 

Perhaps simple weathering, and/or the work of a "boring sponge".

https://natureinfocus.blog/2009/12/05/sponge-borings-in-flat-oyster-shells/

 

you're right, it's boring....

 

I did an advanced marine zoology field course many many years ago. The creatures that live on bivalves and other molluscs are generally symbionts or at worse, just passengers.

I could bore you senseless on the private life of the dog whelk.

 

OMG...I am SO sorry...

 

I knew that was going to happen......