from L. tabula "board, plank," but the Bret. form of this compound would be taolvean. "There is reason to think that this [tolmen] is the word inexactly reproduced by Latour d'Auvergne as dolmin, and misapplied by him and succeeding Fr. archaeologists to the cromlech" [OED]. See
cromlech, which is properly an upright flat stone, often arranged as one of a circle.