id help please

Nice find,  I think it's a sponge, Porosphaera globularis.

thanks bill..i thought it might be an echinoid as appears to have tubercles under eye glass,but maybe its just where its worn away a bit?(couldnt get closer pic)

Tubercles or dimples? Here's a P. globularis close-up.

...and one that didn't clean up very well, with little bits that look like tubercles.

Porosphaera sp. vary quite considerably. See here

You sometimes do find round pebbles contained within a flint, one theory is thet they were gastroliths (stomach stones) of marine reptiles living in the cretaceous sea.  Below are two I have found  , the second being associated with fish scales (to the left) which gives support to this theory.

As I said it is only one possible theory, they equally could be pebbles that existed in cretaceous times that simply became enclosed in flint...but the chalk was laid down in very deep water away from land.  So it is reasonable to attribute the pebbles to regurgitated gastroliths.  As I commented the one I found with closely associated fish scales gives support to this view.  What creature they came from is anyones guess.  I don't think fish use stomach stones so it was probably a reptile, perhaps a Mosasaur given the late age.