These Polynesians of Tahiti, as elsewhere in Oceania, were still living in the "Stone Age" when Cook visited them, but they had already learnt the value of iron from the previous visits of Bougainville and Wallis, and were very keen to obtain iron from the crew of the _Endeavour_. Previous to their intercourse with Europeans, fish hooks were made of mother-of-pearl, or some other hard shell, filed into shape with pieces of coral, drilled with holes by sharp-pointed stones, fixed into the end of a piece of bamboo, which was then rotated between the hands. Coral rock made excellent files for this and other industries. Their axes and adzes were derived chiefly from basaltic volcanic stone. The skin of a sting-ray[74] with its rough tubercles, and used with coral sand, made a fricative for polishing or rubbing down stone surfaces. With their stone axes they were able to fell trees, and, what is more, to split the trunk into planks from 3 to 4 inches thick, the whole length and breadth of the tree, though some of these trees were 8 feet thick and 40 feet long. They would smooth planks very expeditiously with their adzes, and take off a thin shaving from a plank without missing a stroke. [74] The sting-ray is so often referred to by Pacific voyagers that some description of it is necessary. This fish--probably of the genus _Urogymnus_ or _Pteroplatea_--belongs to the family of the _Trygonidæ_, the whip-tailed or sting-tailed rays, so called because the thin, pliable tail is armed with a series of bony spines, as much as 8 or 9 inches long, which have proved very useful to primitive man on the Pacific and Indian Ocean coasts (where the sting-rays are often stranded) as a ready-made lancehead or dagger point. The skin of some species of sting-ray is covered with bony tubercles. The rays or skates are distantly related to sharks, but are specially remarkable for being broad and flat--occasionally broader than they are long--owing to the immense development of the fin flaps at the sides.