The altar has a frontal which is itself concave and decorated with verde antico and alabaster, and protruding from it is a large porphyry urn in the form of a bathtub. Below the altar frontal is a large oval orifice, surrounded by a gilded palm leaf wreath on a grey stone background. This is topped by a pair of curlicues, on which two gilded angels sit. The opening is into the actual confessio or crypt, and is called a fenestrella confessionis. It was intended to aid the devotion of pilgrims.