ST. HILDA'S SNAKE-STONES. 109 



in diameter, while in the Oolites there are species 

 which measure a yard across. 



What the precise position of the Ammonite is 

 in the chain of life cannot be definitely settled. 

 The modifications through which these creatures 

 have passed can only be guessed at by observing 

 minute changes in the 

 shells. Such changes 

 are said to have been 

 observed as proceeding 

 from the outer whorl 

 more and more towards 

 the centre, as geological 

 time rolled on. The 

 earliest Ammonites were 

 smooth, later on they FlQ . 2 6.-A.lautus. 



had ribs, and still later 



spines appeared. The line of descent is usually 

 traced backwards through Goniatites with simpler 

 shell, and onwards, as has already been indicated, 

 to the Nautilus. But the line is confused and 

 broken, and the Nautilus, which is regarded as the 

 lingering scion of a great race, seems destined 

 to die out. 



FIG. 27. A. varicosus and side view. 



