PLATE X. 



Zone of Arietites Bucklandi. 



Fig. 1. Arietites Crossii, Wright. Lateral view, half the natural size, from the 

 Scunthorpe Ironstone, North-west Lincolnshire, remarkable for the great 

 development of the ribs near the line of involution, from the collection of 

 the Rev. J. E. Cross, E.G.S., Appleby, Brigg, Lincolnshire. 

 2. Back view of the same fossil to show the narrow siphonal area, sharp narrow 

 keel, shallow bisulcations, and trenchant ribs. 



