ST. HILDA'S SNAKE-STONES. 



107 



globe long before Ammonites existed, and, indeed, 

 many of its genera had already become extinct 

 before the true Ammonites began to exist ; while the 

 Nautilus still survives, although Ammonites have 

 been absolutely extinct for ages. 



As a help in the identification of specimens it 



FIG. 22.^!. Ufrons. FIG. 23. Side view of A. Ufrons. 



may be as well to present illustrations of a few of 

 the commoner species. 



Ammonites Ufrons (Fig. 22) occurs in considerable 

 numbers in the Upper Lias. Although appearing 

 to characterise one of the zones at Lincoln, this is 

 not found to be the case elsewhere. The shell has 

 a sickle-shaped sculpturing on the sides, and the 

 body chamber occupies about one-half of the first 

 whorl or spiral. 



