occur only at Stony Middleton, Wisby, Milan, Perugia, Ravenna, Hierapolis, and the modern St. Dunstan's-in-the-West, London. There was formerly one at Ayot St. Peter's. The form is mentioned by Eusebius at Antioch in the case of a church built by Constantine, and was a modification of the principle of the round church. There is an octagonal porch at St. Mary's Redcliffe, and a chamber in modern times called the Baptistery, but really connected with the water system, at Canterbury Cathedral.