What is a large oyster looking shellfish?

It's difficult to say definitively without more information! "Large oyster looking" could encompass a variety of different shellfish.

To help narrow it down, tell me:

* Where did you see it? (Ocean, lake, market, etc.)

* What was its shape like? (Round, elongated, bumpy, smooth)

* What was the shell like? (Color, texture, thickness)

* Did it have any other distinguishing features? (Siphons, unusual patterns, etc.)

Once I have more information, I can give you a more specific answer.