See above, p. [45], note, for the original conception of this motive at Orvieto.
See Il Duomo di Orvieto, descritto ed illustrato per Lodovico Luzi, pp. 330-339.
See Luzi, pp. 317-328, and the first extant commission given in 1310 to Maitani, which follows, pp. 328-330.