PLATE XXI A. 



From the Grey Chalk. 



2/ 

 Fig. 3. 

 Fig. 4. 



PsEUDODiADEMA ORMATUM, Goldfuss, fi'oui the Cabinet of the Rev. T. Wiltshire, 

 F.G.S. Upper surface, natural size. (P. 103.) 



Under surface, natural size. 



PsEUDODiADEMA Brongniarti, Affttssiz. British Museum, natural size. (P. Ill .) 



Upper surface, magnified one half diameter 



Under surface, do. do. 



Lateral view, do. do. 



Arabulacral area, poriferous zones, and one 

 half of an inter-ambulacral area, mag- 

 nified six times. 



Primary tubercle, magnified. 



Spines, natural size. 



Portion, magnified six times. 



