Fish scale the surface, interacting only with the riblet tips, not causing any high-veloctiy flow in the valleys. Since this high velocity flow now only interacts with the riblet-tip, which is a very small surface area, the momentum transfer which causes drag is now much lower than before, thereby effectively reducing drag. Also, this reduces the cross-stream velocity fluctuations, which aids in momentum transfer too. The rough, sandpaper-like texture of shark and ray skin, coupled with its toughness, has led it to be valued as a source of rawhide leather, called shagreen. One of the many historical applications of shark shagreen was
Leather carving cutting into the leather and pushing the edges of the cuts in is relatively modern, marking wet leather has been around since the Roman period. There are some beautiful examples of Roman leather at Vindolanda near Hadrian's wall and the book "Purses in Pieces" is very informative with some lovely examples of medieval carved leather work. Historically, tools for stamping leather would probably have been made out of bone, but would not have been too different from what we use today. Three main tools are used in leather carving: This tool, which creates an impression similar to that of a