Pteropoda, a class of Mollusca which live in great numbers at the surface of the ocean far from land in all parts of the world. They have heads, and thus belong to the branch of Mollusca known as Glossophora. They are bilaterally symmetrical, and thus have been regarded as allied to the Cephalopoda; thus Professor Ray Lankester groups them as the Siphonophora, but there is little doubt that this bilateral symmetry has been reacquired, while it is probable that the lateral wing-like processes (or epipodia) are not homologous with the arms of the Cephalopoda. The larvae are always provided with a shell, but in the adult there is one, only in those belonging to the order Thecosomata. This, however, does not represent the larval shell, but one secondarily acquired by all Pteropods; both shells are thrown off in the order Gymnosomata and one family of the other order, viz. the Cymbulidss, in which a third shell is formed; both remain in Styliola, and the larval shell only in the family Hyaleidce. The Pteropods are small animals, and are usually found only at some distance from land, but they have been found on the Scotch coast, as in St. Andrew's Bay; they occur in such enormous shoals that, in spite of their small size, they form a leading part in the food of whales. There are two orders - the Thecosomata, the members of which have a shell, and the Gymnosomata, in which they are shell-less. The group is one of great antiquity, though much doubt arises as to whether the great Conularia, Theca, etc., of the Cambrian rocks are really correctly referred to the Pteropods. Another interesting fossil that has been assigned to this order is Tentaculites from the Silurian, but this is unquestionably a worm.