142 PICTURESQUE SKETCHES 



of the last or outer chamber ; (3) in being frequently 

 covered with numerous bosses, tubercles, ribs, and 

 other projections, which greatly ornament the shell, 

 and of whose use we shall presently speak; (4) in the 

 nature of the wall of separation between each two 

 adjacent chambers ; and (5) in the position of the 

 siphuncle or tube communicating with the various 

 chambers. 



The shell of the ammonite is usually thin, the 

 outer chamber extends for about two-thirds or more 

 of an entire whorl, affording a considerable space for 

 the animal to inhabit ; and the external aperture is 

 often provided with very singular projections, or is 

 swelled out, affording, in all probability, a means 

 of attachment, by which the animal was safely in- 

 closed within its shell. But as this part seems to 

 have been generally thin and fragile, it is rarely 

 preserved in ordinary specimens. 



To make up, perhaps, for the thinness of the 

 shell, and to give additional strength without greatly 

 increasing the weight, most of the different species of 

 ammonites are ribbed and covered with tubercles, 

 which, if we look upon the shell simply as a mecha- 

 nical contrivance for defending a soft animal, and 

 consisting of a continuous arch coiled round itself, 

 served the purposes of transverse arches and domes. 

 The external surface was greatly strengthened by 

 such an arrangement, and the nature of the strength 

 thus communicated it is not difficult to understand. 

 The introduction of the ribs, which were distributed over 

 the surface of the shell transversely, corresponds to 

 the introduction of fluted metal instead of a plane sur- 

 face, often made use of in machinery when it is wished 



