PLATE XVIII. 

 FIST ni CM CASEKTANUM (Poll). 



1. -/<. Internal aspect of a scries of valves from the rejecta- 



menta of the Thames, Chertsey Mead, Surrey. 



2. a, b. of two valves from the Holocene in Berkshire, near 



Staines. 



3. a, b. | of a series of valves from the Holocene at Dagenham, 



4. a-f, J Essex. 



5. a-f. of a series of valves from the Holocene at Gayfield, 



Edinburgh. 



6. of abnormal right valve from the Holocene at Elie, 



Fifeshire, in which one of the posterior lateral 

 teeth is suppressed. 



7. from the Pleistocene of Swanscombe, Kent, in 

 which the lateral teeth are single instead of 

 double. 



8. a, l>. ] of a series of valves from the Holocene at Newbury, 



9. a-f. J Berks. 



10.o,6.1 from the Holocene at Westbury-on-Severn, 



11. a-c. J Gloucestershire. 



12. a-l. from the Pleistocene at Grays, Essex. 



13. a, b. of two valves from the Pleistocene at West Wittering, 



Sussex. 



14. a-li. of a series of valves from the Pleistocene at Clacton, 



Essex. 



15. a-f. from the Cromerian (peat) at West Runton, 



Norfolk. 



*#* Th) scale in millimetres under each figure, or series of figures, 

 shows the amount of magnification. 



