268 



THE WORLD OF LIFE 



CHAP. 



not uncommon. One of the largest English specimens in 

 the British Museum (Natural History) was found at Rotting- 

 dean, near Brighton, and is 3 feet 8 inches across ; but the 



FIG. 99. CERATITES NODOSUS. Trias. FIG. 100. TRACHYCERAS AON. Trias. 



largest known is an allied species from the Upper Chalk 

 of Westphalia, and has the enormous diameter of 6 feet 

 8 inches. 



It is an interesting fact that the very earliest Ammonites 



FIG. 101. CRIOCERAS BMERICI. 

 Cretaceous. 



FIG. 102. HETBROCERAS EMERICI. 

 Cretaceous. 



were straight, and gradually became closely coiled. This 

 form was maintained almost constant throughout the vast 

 periods of the Mesozoic age, till towards the end, when the 

 whole race was about to die out, they seemed to try to go 

 back to their original form, which some almost reached (Fig. 



