DEFENCE AGAINST PRESSURE. 



207 



they terminate in the external shell, they resemble the 

 edges of a parsley leaf. Now, the edges of these 

 plates becoming foliated, as it is called, at their junc- 

 tion with the outer shell, distribute their support more 

 equally beneath all its parts, than if they had been 

 continued as simple curves. 



Section of an Ammonite. 



More than two hundred species of ammonites are 

 known in which strength is gained by this means. The 

 nautili are not thus defended, and why is this ? Most 

 probably because the greater thickness of the shells 

 renders such aid unnecessary. The comparative thin- 

 ness of the outer shells of many ammonites, made more 

 internal support under the pressure of deep water need- 

 ful ; and here it appears. 



