LECTURE XI. 
157 
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- 
^Ijed 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 
