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
Ostrich leather sought after portion and since it constitutes such a small area of the skin, "full quill" ostrich products are considerably dearer when compared to bovine leather. This, along with the fact that it is one of the strongest commercial leathers, leads ostrich leather to be seen as a luxury item. The commercial farming of ostriches first began in the 1850s when pioneering farmers located in Oudtshoorn, South Africa, saw great economic potential in harvesting ostrich feathers. Horse-drawn carriages made large, dramatic hats fashionable. Ostrich feathers are some of the most intricate and grandiose in the world so it only made