Rio de Ca' Foscari, across from San Pantalon.
According to Paolo Giordani in his informative "Venice, Thirty Walks to Explore the City," this building along with the walled arches on Corte del Fontego (Dorsoduro 3421 and 3426, respectively) form a complex that dates from the 9th century. The Gothic façade on the rio must obviously be a later modification to the Byzantine building. Having said that, I must add that I am not sure that Mr. Giordani got the building right. He also adds that Ruskin made drawings of this building when in fact, as far as I know, Ruskin made a drawing of a nearby building that he calls the "Rio Foscari House."

Walled arches on Corte del Fontego

The façade that Ruskin drew can be viewed only from the water, and must correspond, according to my estimate, to numbers 3364 or 3367 also in Dorsoduro. The back of the building, which looks rather modern except for the cut-in-the-middle vera da pozzo, can be seen from Corte de l'Aseo.
Like almost everything in Venice, nothing is what it seems.

Ruskin rendering of what he called "Rio Foscari House"

Back of Ruskin's building on Corte de l'Aseo