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
Russia leather been remarkably hard-wearing and examples have lasted in use into the 19th century, without recovering. When finally worn out, this was often by the surface of the leather having worn right through into a hole, still without any structural cracking of the leather, as a less flexible hide would fail. Russia leather was (and still is) commonly used for production of footwear. In Russia itself, it was usually used to make high boots (sapogi). In this craft, Russia leather was usually divided into two sorts: the more expensive "yalovka" (produced from calf skin) and the cheaper "yuft" (from all other