LECTURE XI. 157 



t 



the principal species of which are inhabited by 

 animals of an appearance so widely remote from 

 that of the rest of the shell-tribe, and so closely al- 

 lied to the genus Sepia or Cuttle, as scarcely to 

 differ except in the circumstance of having two of 

 the arms furnished towards the tip with a very 

 large, expanded, oval membrane, by the assist- 

 ance of which it is enabled to sail along the sur- 

 face of the sea, when calm, in any particular di- 

 rection, and on the least appearance of danger to 

 submerge itself by suddenly contracting its webbed 

 arms, and withdrawing them into the shell. 



The principal species of the genus Argonauta, 

 the first of the Univalves in the Linnsean arrange- 

 ment, is well known to the shell-collectors by 

 the name of the Paper Nautilus* This shell, 

 which grows to a very considerable size, some- 

 times measuring near ten inches in length, is of 

 an appearance uncommonly elegant, representing 

 a kind of boat or vessel, of a slightly compressed 

 shape, gradually widening towards the tip or 

 mouth, and turning up at the back part into a spi- 

 ral curvature. The whole shell, which is scarcely 

 thicker than common paper, of a white colour, 

 and semitransparent, is marked throughout its. 



