PLATE I. 



Zone of Arietites Bucklandi. 



Fig. 1. Arietites Bucklandi, Sowerby. Lateral view. Half natural size. From 

 the Ironstone beds at Scunthorpe, north-west Lincolnshire. My collection. 

 2. Front view of the same Ammonite. 

 8. Enlarged figure of the foliations of the septa. 



4. Arietites semicostatus, Young and Bird. = Arietites geometricus, Oppel. 



Lateral view. Natural size. My collection. 



5. View of the siphonal area, showing the compressed sides and angular keel. 



6. Enlarged figure of the foliations of A. semicostatus. 



7. Young shell, smooth like A. planorbis ; being the four inner whorls of 



A. semicostatus. My collection. 



8. Young shell, in which ribs are developed in early life. My collection. 



