PLATE XL 



PISIDIUM ASTARTOIDES, Saiidbcrger. 



1. a-n. External aspect of a series of valves showing variation in 



the outline and sculpture, from the Cromerian 

 (gravel) at West Eunton, Norfolk. 



2. a-p. from the Pleistocene at Swanscombe, Kent. 



#*# The scale in millioietres under each figure, or series of figures, 

 shows the amount of magnification. 



