LECTURE XI. 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- 

 jnity proceeds a slender tail or process, passing 

 through all the rest of the chambers j and it has 



