256 ZOOLOGY. 



separating nearly allied families ; a circumstance too often 

 observable in the classing of most other natural productions. 

 Our remarks on the last genus (Pinna) will with almost 

 equal propriety apply to the first genus in the present order, 

 •which has the appearance of being composed of two separate 

 sides, but united by a narrow strip or keel of the same tex- 

 ture, which forms a near alliance to the Bivalve shells. 



Most of the shells of this division possess a regular spiral 

 curve, very conspicuous in many of the genera, but gradu- 

 ally becoming obsolete. 



18. Argonauta. Paper Nautilis. Inhabitant a Sepia or 

 Clio. 



Shell entire, spiral, involutCj membranaceous; aperture con- 

 sisting oi one chamber. Plate 5, fig. 18. 



All the Argonauts are marine shells, of exceedingly 

 brittle texture, and possess a great degree of elegance of 

 form. The ancients are said to have derived the art of 

 navigation from the animals inhabiting these shells ; which in 

 calm weather, are seen floating on the surface of the water, 

 with some of their tentacula extended at the sides, whilst two 

 arms that are furnished with membranaceous appendages, 

 serve the office of sails, from whence those beautiful lines ; 



" Learn of the little Nautilis to sail 



Spread the light oar and catch the passing gale." 



These animals raise themselves to the surface of the sea, 

 by ejecting the sea- water from their shells ; and on the 

 approach of danger, they draw in their arms, and with them 

 a quantity of water, which occasions them to sink imme- 

 diately ; by possessing this power they are but rarely taken 



