PLATE XXVI. 



PISIDIUM 'SUPIXUM, A. Schmidt. 



1. , b. Internal aspect of a very triangular pair from Kickmans- 



worth, Hertfordshire. 



2. a-j. of a series of valves of all ages showing variation, 



from the rejectamenta of the Thames at Chertsey 

 Mead, Surrey. 



3. a-f. ) from the Pleistocene at Crayford-Erith, 



4. a-li, \ Kent. 



5. a-d. of two pairs from Marsworth, Buckinghamshire. 



6. , b. of a pair from the Thames at Hampton Wick, 



Middlesex. 



7. Diagrammatic enlargement of the Hinge. 



8. a-d. Internal aspect of two pairs from the Thames at 



Twickenham, Middlesex. 



9. a-d. of four valves from the Cromerian (peat) at West 



Eunton, Norfolk. 



10. a-f. of six valves from the Cromeriaii (gravel) at the 



same locality. 



11. a-b. of a pair from Bud worth Mere, Cheshire. 



12. a-j. of a series of valves showing variation, from the 



Pleistocene at Clacton, Essex. 



PISIDITJM HENSLOWANUM, Sheppard. 



13. a-n. Internal aspect of a series of the thickened form from 



the Pleistocene at Grays, Essex, for comparison with 

 the similar series of P. supinum shown at 4 and on 

 PI. XXVII, 1 & 2. 



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

 shows the amount of magnification. 



