PLATE XXII. 



PlS I DI CM SUBTRUNCATUM, Malm. 



1. -/. Internal aspect of three pairs showing variation, from 



the Six-mile-water, Antrim. 



2. a, b. of a pair from the Avon at Salisbury, Wiltshire. 



3. a, b. of a pnir from Drum Bridge, Lagan, Co. Antrim. 



4. a, b. of a pair from Catford, Kent. 



5. a-y. of three pairs and a left valve from llanmore 



Common, Surrey. 

 G. a, b. of a pair from Khosneigr Anglesey. 



7. #, b. of a pair from Bracebridge, Warwickshire. 



8. , b. of a pair from Lochinaben, Dumfriesshire. 



9. , b. of two valves from the Holocene of the Lea Valley, 



London. 



10. a, b. of four valves from the Pleistocene at West "Wit- 



tering, Sussex. 



11. of a right valve from the Holocene of the Lea 



Valley, London. 



12. of a left valve from the Pleistocene at Swanscombe, 



Kent. 



13. -r, of three valves from the Holocene at Ncwbury, 



Berks. 



14. , b. of two valves from the rejectamenta of the Thames 



at Chertsey Mead, Surrey. 



15. Diagrammatic enlargement of the Hinge. 



16. Posterior and dorsal view of a pair from the Red Bridge 



Pond, Hampstead Heath, London. 



17. of a pair from the Six-mile-water, Antrim. 



18. , 6. External right aspect of two pairs from the lied Bridge 



Pond, Hampstead Heath, London. 



9ft" f/ ' ' i ~~ ^ ^ iree P a i fs f rom Six-mile-water, Antrim. 



21. a-h. External aspect of a series of four pairs from Ran more 



Common, Surrey. 



22. a, b. of a pair from Bracebridge, Warwickshire. 



23. External right aspect of a pair from the Avon at Salis- 



bury, Wiltshire. 



24. of a pair from Catford, Kent. 



25. of a pair from Rhosueigr, Anglesey. 



26. of a pair from Gloucester. 



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

 shows the amount of magnification. 



