I took my 96 Plymouth neon in for a fuel leak today.  It ended up being
an inexpensive fix, but I didn't get a chance to talk to the mechanic.
He left the part for me.

Basically, it's a rubber cap, about the same shape and maybe twice as
big as a tire valve cap.  When I squeezed it, I could see cracks... it
looked like the rubber had dryrot.

Does anyone know what this is?  I can post a picture if necessary.

Sorry if this is incredibly obvious.  I know almost nothing about cars.

Thanks,

Joseph