What is this strange rock?

[babs226]

I found it on a beach in Massachusetts when I was a kid. I identified it once by googling it, but didn't write down the answer and haven't been successful in finding it again. It's driving me crazy. Some kind of septarian nodule or concretion or ??

 

[Back-of-the-boat]

How big is it can you put something with it for size reference.

 

[Back-of-the-boat]

It might be a geode? But to me it looks like a fossilized nut.