Fig. 8
From: From Land to Water: the Origin of Whales, Dolphins, and Porpoises
Cross section of the femur (thighbone) of Indohyus (RR 42). The purple–blue color is fossilized bone in this image taken with a polarized light microscope with a gypsum filter. The marrow cavity is filled with sediment (gray in this image; http://www.neoucom.edu/audience/about/departments/anatomy/AnatFaculty/media, © J.G.M. Thewissen)