Figure above:

Dolmen of Poulnabrone (Ireland) dating from 4,000 to 3,000 B.C. The dolmens are megalithic tombs, consisting of two or more upright stones supporting an horizontal slab, often containing collective burials. In some of the tombs, more than 350 skeletons have been discovered: the presence of a great number of human remains suggest that these monuments were depositories and periodically utilized for communal ceremonies held in honor of the dead. It seems that the dead were venerated not as single personalities but rather as a collective entity, comprising all those who had left the world of the living.
[Image: http://commons.wikimedia.org/wiki/File:Poulnabrone_dolmen-SteveFE.jpg]

Figure below:

Megalithic gallery grave known as La Roche-aux-fées (the Rock of the Fairies), located in the municipality of Essé (Brittany, France). The gallery graves or passage graves represent a monumental development of the dolmen type, the one represented in the picture achieves the length of about 20 meters. The main axis of the monument points to the rising of the sun at the winter solstice.
[Image: http://commons.wikimedia.org/wiki/File:LaRocheAuxFees_Dolmen_2_20070408.jpg]