The ninth book treats of fishes, crabs, sea-urchins, mollusca, and other marine animals, including not only turtles and cetacea, but also mermaids, tritons, and other fabulous creatures. These he arranges in no definite order, although he proposes a kind of classification founded on the covering or skin; some, as seals and hippopotami, having a skin and hair; others skin only, as the dolphin; while the tortoises are covered with a substance resembling bark; oysters and other shells with a substance as hard as flint; echini with crusts and prickles; fishes with scales; sharks with a rough skin fit for polishing wood; lampreys with a soft skin; and polypi with none at all. As might be expected, many wonderful tales are related of the dolphin, which was a special favourite with the ancients, on account of its supposed attachment to the human species. One of these animals, if we may credit Pliny and his authorities, carried a boy daily to school and home again, from Baianum to Puteoli; another, who used to mount a child on his back, having one day suffered him to be drowned, brought back his body, and out of grief thrust himself ashore, where he of course died; and, lastly, a king of Caria having caught a dolphin, and kept him prisoner within the harbour, a whole multitude of the same species came to beg his release, and remained until their prayer was granted. The most interesting chapters in this book are those on pearls and the shell-fish that furnished the purple dye so highly esteemed by the Romans. This oyster, he says, which is the mother-of-pearl, at a certain season of the year, gapes and receives one or more drops of a kind of dew, which are ultimately converted into pearls. According to the nature of this dew, or the state of the weather at the time of its being received, the pearl is dusky or white, dull or possessed of a brilliant lustre. These ornaments were very highly esteemed in Pliny's days. The ladies wore them dangling at their fingers and ears, took great delight in hearing them rattle, and not only appended them to their upper garments, but even embroidered their buskins with them. It will not suffice them, says he, nor serve their turn, to carry pearls about them, but they must tread upon pearls, go among pearls, and walk as it were on a pavement of pearls. Lollia Paulina, the wife of Caligula, was seen by him, on an ordinary occasion, ornamented with emeralds and pearls, which she valued at forty millions of sestertii (about L300,000).