A stapelia variegata

Around 1641, Nicolas Robert executed for Gaston Duc d'Orléans (1608-1660) a large number of watercolors on vellum of flowers and plants. After Gaston's death, Robert worked for King Louis XIV and was appointed Peintre ordinaire du Roi in 1666. The most important collection of Robert's drawings on vellum is at the Natural History Museum in Paris, with more than six thousand watercolors (see Nicolas Robert et les Vélins du Muséum National d'Histoire Naturelle, Paris, 1980). Drawings of this type are distinctive through their gold borders (about 3-5 mm.), and the numbers in the lower right corner.
Another vellum representing a stapelia by Robert, but this time a stapelia gettleffii was sold, Sotheby's, London, 7 July 1999, lot 150.