222 THE STORY OF THE EAKTH AND MAN. 



chambered like it, so as to serve as a float, but 

 far more elaborately constructed, inasmucb as tlie 

 chambers were not simply curved, but crimped and 

 convoluted, so as to give the outer wall much more 

 eSectual support. This outer wall, too, was worked 

 into ornamental ribs and bands, which not only 

 gave it exquisite beauty, but contributed to com- 

 bine strength to resist pressure with the lightness 

 necessary to a float. In some of these points it 

 is true the Gyroceras and Goniatites of the False - 

 ozoic partially anticipated them, but much less per- 

 fectly. The animals which inhabited these shells 

 must have been similar to that of Nautilus, but 

 somewhat difi'erent in the proportion of parts. They 

 must have had the same power of rising and sink- 

 ing in the water, but the mechanical construction 

 of their shells was so much more perfect rela- 

 tively to this end, that they were probably more 

 active and locomotive than the Nautili. They must 

 have swarmed in the Mesozoic seas, some beds of 

 limestone and shale being filled with them; and 

 as many as eight hundred species of this family 

 are believed to be known, including, however, such 

 forms as the BacuUtes. or straight Ammonites, bear- 

 ing to them perhaps a relation similar to that of 

 Orthoceras to Nautilus. Further, some of the Am- 

 monites are of gigantic size, one species being 

 three feet in diameter, while others are very minute. 

 The whole family of Ammonitids, which begins to 

 be in force in the Trias, disappears at the end of 



