The univalve shells, as we have previously explained, are the calcareous cases, or coverings, of a more highly organized class of molluscous animals, than the inhabitants of the bivalves (see p. 366), for they possess a head and mouth with jaws, eyes, and feelers; and while the _Acephala_, with but few exceptions, are incapable of locomotion, the _Encephala_ are almost all of them furnished with organs of progression, and can creep, climb, and swim, or float on the surface of the water. Their shells are for the most part formed of one valve, hence the name of _univalve_; but in some species it is composed of several pieces. The most simple form of shell is that of the hollow cone, of which the _Patella_, or limpet, affords an example; and in the more complicated modifications, the cone is twisted, or convoluted spirally, either in the same plane as in the _Planorbis_ of our rivers, or obliquely, as in by far the greater number of species. The direction of the spire is generally from left to right, the aperture being dextral to the observer when the shell is placed with its apex uppermost, as in _Lign. 133, figs. 1, 2, 3_; but in a few species the spire turns in the opposite manner, and the mouth or aperture is to the left, or sinistral, as in _Lign. 133, fig. 4_. In consequence of the form of the aperture of the shell, the entire or notched condition of its margin, and the presence or absence of a canal or siphon always having relation to the soft parts of the animal, these characters afford data by which the genera and species of the shells may be determined, and information obtained as to the structure and economy of the originals. The Gasteropoda generally creep by means of a fleshy disk, or foot, which is situated under the belly. Some kinds are terrestrial, others inhabit trees, many live in rivers and streams, others in stagnant and brackish waters; but the greater number are denizens of the sea. [Sidenote: FOSSIL GASTEROPODA.]