That's the exact shape for a pipe lid. Yours has been squished of course, but it's a half-sphere when it's not malformed and it covers the opening of old pipes. I did some research and learned that pipes started getting fancy around the 1700s, with designs, lids, etc, and that pipes have had lids ever since. I would guess (and I really have no idea what I'm talking about) that your pipe lid might be from the mid to late 1800s. Take that with a very large chunk of salt