PLATE XIV. 



PlSIDItlM CASEKTANUM (Poll). 



1. a, b. External aspect of two opposite valves from the Holocene 



at Gayfield, Edinburgh. 



2. of left valve from the Cromerian (peat) at "VYesfc 



Runton, Norfolk. 



3. a-e. of a series of left valves from the Holoceue at 



Dagenham, Essex. 



4. a-x. of a series of valves showing variation, from the 



Pleistocene at Swanscombe, Kent. 



5. (t-l. - from the Pleistocene at Grays, Essex. 



6. a-e. - from the Holocene at Newbury, Berkshire. 



7. a-p. from the Pleistocene at Crayford-Erith, Kent. 



8. a-Ji. from the Pleistocene at Clacton, Essex. 



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

 shows the amount of niao-nification. 



