The AQUATIC, or Fresh-water Molluscs, such as the Planorbis, commonly called the Fresh-water Snail; the Unio--known by the name of Fresh-water Muscle, is found in ponds, ditches and rivers. The _epidermis_ of these is generally of a thick, close-grained character, and they are subject to corrosion near the umbones. There are but few genera of fresh-water shells besides the Uniones, among bivalves, and the "Melaniana" among univalves. Concerning the former it may be observed, that they are all pearly within, and the colour of the thick horny coating embraces all the varieties of brownish and yellowish green. The MARINE, or _sea-shells_, belong to all the classes and orders, and include by far the greater number of species. They vary in the habits of the animal, and consequently in the situations in which they are found. Some are found buried in sand and marine mud, and are named "_Arenicolæ_" or inhabitants of sand; others in holes of rocks and other hard substances, then they are named "_Petricolæ_,"--some of these latter form the holes in which they live by corroding or eating away the stone. A section of these form the family of "_Lithophagidæ_," or stone-eaters, of Lamarck. Others, again, take up their parasitical abode in the bodies of animals, and feed upon their substance; as for instance, the Stylifer, which is found in the vital part of star-fish, and Coronula, and Tubicinella, found buried in the skin of the whale. LOCOMOTION--_Attached, Unattached._ A much more subordinate source of distinction arises from the freedom or attachment of the shells. Some of them float or walk freely in their natural element; others are fixed or attached to foreign bodies. Among those which are attached, there is again a difference as to the mode of attachment. Some are united to foreign bodies by means of a glutinating substance, secreted by the animal, and joining part of the surface of the shell to that of the stone, coral, or other substance. In this way shells are fixed to each other in groups; this is the case with the Spondyli among bivalves, and the Serpulæ among univalves. M. de Blainville applies the term "_Fixæ_" to these shells. Others are kept in a particular place by means of a _Byssus_ or Tendinous fibrous line or bunch of silky hairs, acting as a cable, and allowing the Mollusc to ride as it were at anchor. This Tendon is connected with some part of the animal from which it passes through an opening or hiatus in the shell, as in the Terebratula and the Mytilus. [Illustration] In the former, represented by the cut, fig. 8, the tendon passes through a perforation in the upper valve; and in the latter, Mytilus, fig. 9, the byssus passes out between the valves.