Top row: Physa gyrina (U. S.) Melania tetrica (Viti Islands.) Angitrema verrucosa (U. S.) Second row: Planorbis trivolvis (U. S.) Third row: Pleurocera elevatum (U. S.) Ampullaria depressa (U. S.) Limnaea stagnalis (U. S.) Bottom row: Vivipara contectoides (U. S.) Campeloma subsolidum (U. S.) Limnaea megasoma (U. S.) The Limnaeas of which we have been speaking have mostly dextral or right-handed shells, that is, have the aperture on the right side when you hold the shell in the position pictured on our plate. In the family Physidae the shell is left-hand or sinistral. The members of this family have shining, horn-colored shells, more or less fusiform. The tentacles are long and filiform and are constantly moving about as in the allied genus Planorbis. The animal is very active and moves about with a steady, gliding motion. It is very interesting to watch a number of Physae in an aquarium; as they are crawling along the bottom, one will be seen to rise suddenly to the top of the water and move along with the foot applied to the surface, the shell hanging down. Again, they may be seen descending, suspended by a thin thread of mucus. When the animal rises suddenly, the branchial cavity which contains the lung is heard to open with a faint, clicking sound, which is probably due to the pressure of air in the lung being suddenly liberated. Several of the species of Physa inhabit water as cold as the freezing point and they may be frequently seen in winter gliding over the bottom of a stream or pond when the surface is frozen. The little glairy, transparent masses of jelly-like matter which are seen attached to stones and the under side of sticks, are the eggs of Physa or Limnaea. Not all of the fresh-water pulmonates have spiral shells. A whole family, the Ancylidae, have a conical shell formed like a rounded shield, and resembling the limpets, hence called the river limpets. They are generally quite small and live attached to the interior of dead river shells and to submerged plants and to rocks. They are very interesting, but hard to find on account of their small size and inconspicuous habitat.