PLATE XXIII. 



PISIDIUM PULCHELLUM, Jenyns. 



1. ff, b. External right aspect of two pairs from Comber, Co. Down. 



3. of a pair from Aldenham, Hertfordshire. 



4. of a pair from Denham, Buckinghamshire. 



5. of a pair from Harefield, Middlesex. 



6. a, b. Posterior and dorsal view of a pair from the same locality. 



7. a? ,. of a pair from' Bracebridge, Warwickshire. 



8. a, 6. of a pair from Walworth, Co. Londonderry. 



9. , b. External right aspect of two pairs from the same locality. 



10. of a valve from the Holocene of the Lea Valley, London. 



10*. of a valve from the Holocene in Berkshire, near Staines. 



11. a-d. of a series of pairs from Bracebridge, Warwickshire. 



12. a-c/. Internal aspect of another series from the same locality, 



13. Diagrammatic enlargement of the Hinge, 



13*. External right aspect of a valve from the Pleistocene at West Wittering, 



Sussex. 



14. a-f. Internal aspect of a series of valves from the Holocene of the Lea Valley, 



London. 



15. , b. of a pair from Comber, Co. Down. 



16. a, b. of a pair from Walworth, Co. Londonderry, 



17. a, b. of two valves from the Holocene in Surrey, near Staines, 



19. a, b. of a pair from the Coin, West Drayton, Middlesex. 



20. a, b. of a pair from Denham, Buckinghamshire. 



PISIDIUM WLLJ^soRGfi, Clessin. 



2. External right aspect of a pair from Silverdale, I # ai}cashire, 



18. a, b. Internal aspect of another pair from the same locality. 



[These figures are placed here for comparison with those of the 

 foregoing species for which P. lilljeborr/ii has sometimes been mistaken.] 



PISIDIUM HEMSLOWANUM, Sheppard. 



21. External right aspect of a pair from the Avon at Salisbury, Wiltshire. 



22. a-c. External aspect of three pairs from the Wendover Canal, Buckingham- 



shire. 



23. a, b. External right aspect of two pairs from Sutton Coldfteld, Warwickshire. 



24. of a pair from Berkhampstead, Hertfordshire. 



28. a, b. External aspect of two very young valves from the Holocene at Blyth- 

 burgh, Suffolk, to show the position at this age of the 

 appcndiculae, (Of. also 29 c & d.) 



^P, a, d. \ Posterior and dorsal views of apposed valves from the Pleistocene at 



^' b, c. J Crayford-Erith, Kent. 



27. a, b. of a pair from the Red Bridge Pond, Hampstcad Heath, London. 



27. c. External right aspect of another pair from the same locality. 



28. a. b. Posterior and dorsal view of a pair from Baguley, Cheshire. 



29. a-t. External aspect of a series of valves of all ages showing variation, from 



the Pleistocene at Crayford-Erith, Kent. 



[s & t are examples of the fractured and repaired valves which are 

 not uncommon in the deposit.] 



30. Or-j. of a series from the Pleistocene at Swanscombe, Kert, 



31 . a-k. of a similar series from the Pleistocene at Clacton, Essex. 



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

 .-hows the amount of iimsrnificatiou. 



