The polygonal formations observed in the walls of Etruria belong to the Pelasgic civilization, and are similar to those of Greece and of Asia Minor. The commercial relations existing between the Etruscans and the Hellenes of Greece and Magna Graecia account for the existing similitudes in their artistic productions. The ceilings of the hypogea, hewn so as to represent caissons, tend to corroborate the idea that their earliest structures were of wood, which, with them as with the Greeks, became the archetype of their structures in stone. To the Etruscans the invention of the arch, constructed on its true principles, has been generally attributed, as likewise the composition of an order styled Tuscan, a species of simple Doric, no entire example of which, however, has been bequeathed to us by the ancients.