|Mermaid from Egenulf, 1527|
I hit the jackpot with search terms earlier. Sirena Bicaudata brings up some wonderful flickr pools of historical mermaids. There's a gorgeous series of them from a 12th century Swiss church that I'm in love with. Not to mention a 1513 siren by Durer that is totally fabulous (keep scrolling in the first link.) This group of images, many of them architectural, is also tons of fun to go delving into.
I also found a siren pattern in one of my favorite modelbuchs, Christian Egenulf's 1527 Modelbuch aller art Nehewercks un Strickens. I love the goofy book because it is so very hand drawn. It doesn't have the slicker more etchy look of so many of the other books. Nor anything as formal as grid lines. It is inspiring me to simplify the mermaids for the motifs and to just go for it. The date is also helpful for my early documentation.
And just to throw in one more design inspiration and make sure we keep it 16th century Italian, despite my drooling over sirens hither and thither and yon, here's a roundel from a print made in 1460-70's Florence. It's held in the British Museum and the curator's comments state that it was probably reference for ivory or wood carvers.