I take it you mean this one:

http://www.beyond-the-pale.co.uk/frenchgenius.htm

about half way down page. It's a beauty.

There's one in the dried up moat of a Paris museum too.

Oh and the thing in the zoo is not a dolmen - it's a "megalithic kist" (or cist if you prefer). Sorry to be pedantic, but dolmens are French things. It's become far too generic a term. I heard someone call Newgrange a dolmen the other week!

And I do know the Welsh word is similar to the Breton one too :-)