PLATE XVII. 



PlSJDIUM CASERTANTTM (Poll). 



1. a-o. ] Internal aspect of a series of valves showing variation, 



2. a-o. J from the Pleistocene at Crayford-Erith, Kent. 



[ from the Pleistocene at Swanscombe, Kent. 



4. a-v. J 



[3 q & s and 4 p are abnormal, partly reproducing 

 the characters of the opposite valve. The two formei 

 having each a single anterior lateral tooth instead of 

 two ; whilst the last has a double instead of a single 

 anterior lateral.] 



*jif* The scale in millimetres under each figure, or series of figures, 

 shows the amount of magnification. 



