While the oyster shell is completely fixed to its bed, the Pecten is, on the contrary, perfectly free, and shifts from place to place, moving in the water even with a certain amount of agility; by smartly closing its half-opened valves and forcibly expelling the water, it moves backward by a sort of reaction; this action, repeated many times, compels the animal to move almost in spite of itself, and enables it to avoid danger, or directs its steps towards the spot it wishes to reach. Some naturalists even assert that, when raised to the surface, the Pecten half opens its shell in such a manner that the upper valve serves the purpose of a sail. [Illustration: Fig. 177. Pecten opercularis (Linnæus).] The Pectens, of which a hundred and seventy-six species are described, are inhabitants of every known sea. Twenty species belong to Europe, among which we may mention _P. opercularis_, represented in Fig. 177; _P. glaber_, and _P. nivea_. Fig. 178 represents the White-mantled Pecten (_P. plica_, Linn.) of the Indian Ocean, and Fig. 179, the Concentric Pecten (_P. Japonica_) of the Japan seas. [Illustration: Fig. 178. Pecten plica (Linnæus).] [Illustration: Fig. 179. Pecten Japonica (Gmellin).] Among the Ostreadæ the shells of _Spondylus_ are distinguished for their variety of form and the brilliant colours with which they are decorated. This makes them much sought after by amateur collectors, and procures for them a high price. The shell of _Spondylus_ is solid and thick, with unequal adherent valves, nearly always bristling with spines, forming a very peculiar kind of ornamentation to the valves; the hinges have two very strong teeth. The animals which inhabit this shell resemble the oyster in many respects, but they still more closely resemble the Pectens. The edges of the mantle are provided with two rows of tentacles, the exterior row being, many of them, furnished at their extremities with coloured tubercles. As examples, we note several species of these bivalves for representation. _Spondylus regius_ (PL. XV. Fig. I.) is, perhaps, the most remarkable for its immense spines. _Spondylus radians_, Lamarck (Fig. III.), is noted for its elegant form. _Spondylus avicularis_ (Fig. IV.) shows remarkable inequality in the valves. _Spondylus imperialis_, Chenu (Fig. II.), has long projecting spines, like feet, and the Scaly Spondylus (_S. crassisquama_, Fig. V.) is covered with scales arranged like so many roofing-tiles. Like oysters, the genus Spondylus is frequently found firmly rooted to rocks and other submarine bodies, and, oftener still, heaped one upon the other, like herrings in their barrel. These animals belong essentially to the seas of warm countries. We find them, however, occupying considerable space in the Mediterranean, where (Fig. VI.) the Ass-footed Spondylus (_S. gæderopus_) abounds.