This seems to be a reference to Palazzo Ariani-Minotto-Cicogna at Dorsoduro 2378, Nadali & Vianello (1999) Tav.38. Sheet No. 156 [n/a], now in Smith College Museum of Art, has a drawing of tracery clearly of Palazzo Ariani-Minotto-Cicogna and called by Ruskin ‘Tracery at San Sebastiano’. Compare the references at Notebook M2 p.4L and Notebook M2 p.35, and see Works, 10.309 for the tracery and Works, 11.274 and illustration of Sheet No. 157 at Plate XVIII facing Works, 38.294 [n/a] for its capitals.
See here.
[Version 0.05: May 2008]