48 LINN^AN GENERA. 



it is by throwing out a quantity of water, by which it becomes speeifi. 

 cally lighter than the element in which it lives, or by taking in a quan- 

 tity of ail-, which will produce the same eflfect. It is only when the sea 

 is calm and imniffled, that the Argonauta, with his feeble bark, ap- 

 pears on the surface. In rising through the water, the shell is reversed, 

 the shai-p edge of the keel presenting less resistance to the liquid, and 

 when it reaches the surface, the animal, by exserting its arms, restores 

 it to a proper position for its voyage. A quantity of water is taken 

 into the shell to balance it ; the animal then employs its arms as oars ; 

 or if a gentle breeze sweep the surface, it erects an ovate extensible 

 membrane, at the extremity of the tentacula, in the manner of a sail, 

 while its arms serve as oars to direct its course, or to keep the shell 

 steady, as well as part of the body which hangs over the shell and 

 serves to act the part of a rudder. Thus equipped, the solitary navi- 

 gator, in his little galley, impelled by the breeze, glides smoothly along 

 the bosom of the ocean. But on the approach of the smallest danger, 

 it instantly retires within its shell, and, taking in a quantity of water, 

 or ejecting a volume of air, quick as thought it sinks to the bottom. In 

 fine weather this shell is frequently to be met with in the Mediter- 

 ranean and has obtained amongst seamen the name of " Portuguese 

 man-of-war," under which name, however, are confounded two or more 

 very distinct animals. 



Genus 19.— NAUTILUS. 



Animal unknown. Shell univalve, divided into several 

 compartments, communicating with each other by a siphun- 

 culus or aperture. 



The Linnsean Nautili are divided into two families : * spiral and 

 rounded, ** elongated and straight. Of the former there are two 

 sections: t whorls contiguous, ft whorls separated. 



Nautilus Pompilius. — The Pearly Nautilus. Plate 

 VII. fig. 19. With the aperture heart-shaped, the spire 

 involute and concealed, pale yellow with mixed streaks and 

 undulations of bright chestnut; aperture pearly and shining, 

 within from five to eight inches in diameter. Inhabits the 

 East Indian ocean. Plate III. fig. 3, represents this shell 

 with the lip cut straight, and a piece removed from the 

 side, to show the involuted spire, c aperture, t involuted 

 spire. 



Nautilus Spirula The Spiral Nautilus. Plate III. 



fig. 11. (Spirula Peronii, Lamarck.) Aperture of the 

 shell orbicular; whorls cylindrical, separated, and gradually 

 decreasing inwards, the first a little straight; siphunculus 



