TheUnivalve shells of the Swedish naturalist, a term adopted from Aristotle's Monothyra, are next to be considered; these, with the multivalve Chitons, form the Gastropods, or shell-fish using their belly for a leg, of Cuvier; and with the cuttle-fish and nautilus tribe constitute Lamarck's Class of Molluscans. The latter author divides his Class into five orders, four of which belong to the tribe I am considering.

1. Pteropods (wing-footed); furnished with organs only for swimming and sailing.

2. Gastropods (belly-footed); body straight, never spirally convolved; a muscular foot for creeping under the belly.

3. Trachelipods (neck-footed); greatest part of the body spirally convolved, always inhabiting a spirivalve shell; foot free, attached to the neck, formed for creeping.

4. Heteropods (diverse-footed); no coronet of arms; no subventral, or subjugular foot; fins, one or more, not disposed in pairs.

As the Cephalopods, forming Lamarck's fourth Order, may be regarded rather as constituting a larger division or Sub-class of the Molluscans, than an Order, I shall consider them in a separate chapter.

1. Proceeding from one of the above Orders to another, I shall select such individuals, belonging to it, as appear to exemplify the great attributes of their Creator, either in their structure, forms, habits, or instincts. The animals of the first Order, like the long celebrated Argonaut and Nautilus, enliven the surface of the ocean in fine weather, where they steer their little barks through, between, and over its fluctuating waves, and spread their membranous sails to the soft breathing of the zephyrs.