PLATE XXVIII. 



PlSIDIUAI STBENBUCHII (Mollel). 



2. External right aspect of a pair from Abbey Holme, 



Cumberland. 



- of a pair from Bjerkiiig, M&lselvdalen, Norway. 

 ("P. ylobulare, Clessin.") 



4. Internal aspect of a left valve from Abbey Holme, 



Cumberland. 



5. External right aspect of a pair from Grisel Bottom, 



Burwell Wood, Lincolnshire. 



6. of a pair from West Greenland. 



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



8. a, b. of a pair from Grisel Bottom, Burwell Wood, 



Lincolnshire. 



10. , b. Internal aspect of a pair from West Greenland. 



11. a, b. of a pair from Grisel Bottom, Burwell Wood, 



Lincolnshire. 



14. Diagrammatic enlargement of the Hinge. 



15. a, b. Internal aspect of a pair from Bjerking, Malselvdalen, 



Norway. (" P. ylobulare, Clessin.") 



16. a, b. of a pair from Finmarken, Norway. ("7\ ylobu- 



lare, Clessin.") 



PlSIDITTM LILLJEBOKGII, Clessin. 



1. a~c. External right aspect of three pairs from Lochan a' Chait, 

 Perthshire. 



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



locality. 



.jo' ' 7 '[ Internal aspect of two pairs from the same locality. 



17. a-c. External right aspect of three pairs from Sonset, Norway. 



(Clessin.) 



18. a, b. Posterior and dorsal view of a pair from Lough Aguse, 



Co. Fermanagh. 



19. a, b. of a pair from Sonset, Norway. (Clessin.) 



20. a-d. External aspect of four valves from the Holocene at 



Gayfield, Edinburgh. 



21. a-d. of four valves from the Pleistocene at Crayford- 



Erith, Kent, 



22. a-j. Internal aspect of five pairs showing variation, from 



Sonset, Norway, (Clessin.) 



23. Diagrammatic enlargement of the Hinge. 



24. a-d. Internal aspect of four valves from the Holocene at 



Gayfield, Edinburgh. 



25. a-f. of three pairs from Lough Aguse, Co. Fermanagh. 



26. a-f. of six valves from the Pleistocene at Crayford- 



Erith, Kent. 



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

 the amount of magnification, 



