UNIVALVE SHELLS. 41 



amongst the ancients, and frequently mentioned in the writings of Pliny and 

 others. 



The singular structure and wond<^rful economy of this animal very early 



attracted the attention of naturalists. T<j its progressive motion on the 



surface of the ocean mankind are indebted, it is said, for the first hint of 



using sails in navigation. This is alluded to by Pope, in the following lines : 



Learn of the little Nautilus to sail. 



Spread the thin oar, and catch the driving gale. 



What the particular organization is which enables this animal to rise to 

 the surface, or to sink to the bottom of the ocean at pleasure, has not yet 

 been satisfactorily accounted for by naturalists ; whether it is by throwing 

 out a quantity of water, by which it becomes specifically lighter than the 

 element in which it lives, or by taking in a quantity of air, which will pro- 

 duce the same eflFect. It is only wlien the sea is calm and unruffled that 

 the Argonauta, with his feeble bark, appears on the surface. In rising 

 through the water, the shell is reversed, the sharp edge ol the keel present- 

 ing less resistance to the liquid, and when it reaches the surface, the animal, 

 by exerting its arms, restores it to a proper position for its voyage. A quan. 

 tity of water is taken into the shell to balance it; the animal then em- 

 ploys 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 man. 

 ner 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 navigator, 

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

 of the ocean. But on the approach of the smal'est danger, it instantly re- 

 tires 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 Mediterraoean, and has obtained amongst 

 seamen the name of a Portuguese man-of-war 



Genus 19 NAUTILUS. 



Animal unknown. Shell univalve, divided into several com- 

 partments, communicating with each other by siphunculus or 

 aperture. 



The Linnean Nautili are divided into two families, *spiral and rounded, 

 **elongated and straight. Of the former there are two sections : fwhorls 

 contiguous, f fwhorls separated. 



Nautilus pompilius. — The Pompilius 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. 



d3 



