AMMONITES. 203 



Like a nautilus, an ammonite has an external shell, 

 a series of internal air-chambers, and a siphon passing 

 through them to the innermost extremity of the shell. 

 The place and use of these shells have caused much per- 

 plexity ; but there is good reason to believe that they were 

 entirely external, and that the position of the body of the 

 animal was within, like that of the nautilus, page 175. 



It is worth while dwelling on the admirable adapta- 

 tion of these shells to their circumstances. As they 

 were to act as floats, they must be thin, or else they 

 would have been too heavy to rise to the surface ; but 

 they were also to afford protection, and thus they must 

 be made strong, to resist the pressure of the water at 

 the bottom of the sea. The provision in the latter case 

 is as complete as it is in the former. 



That this may be clearly understood, it will be desir- 

 able to notice the depth of the ocean, and the pressure 

 of its waters. As to the depth of the sea our informa- 

 tion is very imperfect, and perhaps accurate information 

 is not very likely to be obtained. Still, there has been 

 a sounding of six thousand feet in the Caribbean Sea ; 

 and a line of four thousand six hundred and eighty feet 

 did not reach the bottom of the Northern Ocean. Now 

 far short of these depths the pressure of the water is 

 very great. If a bottle containing only air be tightly 



