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
Jewish views of leather use leather is principally employed among nomads, as it was among the ancient Israelites, since receptacles of leather are not liable to be broken and are easily carried about. The original form of a table, as the word indicates, was a piece of leather, which was spread upon the ground. Pails and all other vessels for holding liquids were made of leather. The leather bucket for drawing water out of a well and the leather flask—consisting of a single skin removed from the animal's carcass as intact as possible—for holding wine or for transporting water have remained in common use