(1) Those in stone, with plain sloping roofs, and without inscriptions, are the oldest.
(2) In 1160 these plain prismatic roofs began to be ornamented.
(3) In the same century the sloping roofs gave place to armorial bearings.
(4) In the thirteenth century we see flat roofs, and figures carved on the lids.
(5) The next stage was an arch, built over the monument to protect it.