The altar may also be marked with a surmounting ciborium, sometimes called a baldachin. The main altar carved ciborium or canopy (1667) by Carlo Rainaldi. In churches, a ciborium is usually kept in a tabernacle or aumbry. The altar was protected by a canopy or "ciborium" resting on pillars. The ciborium in the apse is made of alabaster and lapis-lazuli. The shed roof is a ciborium over an altar. The ciborium of the main altar was built in 1547 by Jacopo Barozzi da Vignola. In the central presbytery, the bronze ciborium was designed by Ciro Ferri in 1681. In the spire at the top of the ciborium, stands Christ surrounded by four angels. A reference to "Presbyter Mercurius" is found on a fragment of an ancient ciborium. Above the statue, a Gothic ciborium was inserted into the southern side wall. The ciborium seems to have been a symbolic tomb. The ciborium was hexagonal and made of wood and silver. The ciborium is supported by four massive red granite columns. Saint Andrew is pictured on the left of the ciborium.