It’s a surprise (Susan stop reading!).
This is based on an image that we saw in (I think) Time magazine in the late 1960s. It contained two figures of reversed mermaids, i.e. with the upper part fish and the lower part human. I didn’t recall the painter or title. After many years of (sporadically and half-heartedly) searching, I finally found the original image in November 2015.
Rights to the original image are apparently owned by the Museum of Contemporary Art, Chicago: Gifted by Joseph and Jory Shapiro, 1982. The painter is Rene Magritte. The title is Merveilles de la nature (1953), translated as The Wonders of Nature. There are many images online of poorly-done replica paintings. See: http://www.renemagritte.org/the-wonders-of-nature.jsp
Previously, I called this project Merfolk (abbreviated MF); now that the title and painter of the original painting are known, I am retitling the project Wonders of Nature (abbreviated WN).
I intend to model the piece in Blender, have it 3d-printed by Shapeways, and then paint it to match the stone look of the original painting. It turns out that the sailing ship in the background of the image is also in another Magritte painting, which could be printed on card stock, and propped behind the sculpture. It might be nice to cover the sculpture with a clear dome, to keep dust off.
2016-02-01: The MakeHuman project has 3d models of the human figure, which can be imported into Blender. I have imported, sized, and posed the lower limbs for the two figures.
2016-03-05: Re-imported human figures from MakeHuman, and posed them for sculpting. Began experimenting with fish models.