"My first genus is very beautiful," replied Mr. Elliot; "it is _Argonauta_, or paper-sailor. The shell is univalve, involute, unilocular, or without chambers: the aperture cordate. (Plate 6.) These shells are spiral, and remarkably brittle. The argonauts are supposed to be the shells that taught mankind the use of sails in the earliest ages of society. In calm weather the animal rises with its shell to the surface of the water, and spreads its arms over the edge; these arms answer the purpose of oars. It then spreads a membrane for a sail, which can be turned in any direction, and is impelled forwards by the breeze: two other arms serve as rudders to direct the course. The animal first raises itself to the surface of the sea by ejecting a quantity of water; if danger occurs, it absorbs water, and thus, by making itself heavier, sinks to the bottom. The species are few in number. [Sidenote: NAUTILUS. CONUS.] "_Nautilus_, pearly sailor, has several characteristics of _Argonauta_; but the former is concamerate, the latter without chambers in the shell. The generic characters of _Nautilus_ are, shell univalve, divided into several compartments, communicating with each other by an aperture. _Nautilus pompilius_ is often cut through, or bisected, to display the chambers of the shell. In the East, the shells are formed into drinking-cups. Sometimes the outer coat of the shell is removed, and the pearly surface finely carved. This genus, according to Linnæus, consists of fifty-eight species, some of which are fossil. (Plate 6.) "In the following genera we must pay particular attention to the _aperture_ of the shell, which is a generic distinction in most univalves. [Illustration: Plate 6.] "The first is _Conus_, a large and beautiful genus, including many rare and valuable species. Shell univalve, turbinate, aperture effuse, or having the lips separated by a sinus, linear, without teeth, pillar smooth. In their natural state the shells are usually covered with an epidermis; but will bear a brilliant polish. _C. textilis_, cloth of gold, is valuable. _C. generalis_ is sometimes sold for twenty guineas. The example on the table is _C. Ebræus_, or Hebrew cone. (Plate 6.) Species one hundred and fifty-five; not one British. The greater number are brought from the Indian Ocean; some from the seas of Africa and from the South Sea. [Sidenote: CYPRÆA. BULLA.] "The shells of the genus _Cypræa_, cowry, are general favourites: the species are fifty-eight; one British, _C. pediculus_. _C. moneta_ (Plate 1) is very common. The generic characters are, shell univalve, involute, obtuse, smooth; aperture linear, the whole length of the shell; effuse at both ends, toothed on each side.