LECTURE XL 
165 
often measuring five or six inches in length : it is 
of a very firm or dense fabric, of a smooth, round- 
ed outline, and of a shape somewhat compressed 
on the sides, with a very wide opening or mouth, 
and with the back-part rolled into a spiral form 
within the cavity of the shell. The colour, exter- 
nally, is a dull yellowish-white, marked with nu- 
merous zebra-like yellowish-brown or dusky bands, 
and. within of the richest and brightest silvery- 
pearl-colour. When the natural pellicle or epi- 
dermis of the outside is rubbed off, the whole shell 
appears silvery also. The great and striking cha- 
racter of the genus however, at least so far as re- 
gards the shell, is the extraordinary structure of 
the internal part, which is formed into a great 
number, (from thirty to forty) separate chambers 
or divisions, each communicating with the rest by 
a small tubular hole near the centre. The open- 
ing or mouth of the shell therefore presents a large 
but shallow concavity, pierced with a central or 
nearly central hole, and beyond lie all the divi- 
sions before-mentioned. The body or chief part 
of the inhabiting animal fills up the front or great 
concavity, and that only ; while from its extre- 
mity proceeds a slender tail or process, passing 
through all the rest of the chambers ; and it has 
