PLATE XXV. 



PISIDIUM SUPINUM, A. Schmidt. 



1 . a, &. 1 Posterior and dorsal view, of two pairs from the Thames 



2. a, b. S at Twickenham, Middlesex. 



3. a, b. of a pair from Rickmansworth, Hertfordshire. 



4. , 6.1 of apposed valves from the Pleistocene at Grays, 



5. , b. / Essex. 



6. , b. of a pair from the same formation and locality. 



7. a, b. of apposed valves from the Pleistocene at Crayford- 



Erith, Kent. 



8. a-h. External aspect of a series of all ages showing variation, 



from the Pleistocene at Clacton, Essex. 



9. a-n. from the Pleistocene at Crayford-Erith, Kent. 



10. a, b. External right aspect of two pairs from Marsworth, 



Buckinghamshire. 



11. of a very triangular pair from Rickmansworth, 



Hertfordshire. 



12. a-c. of three pairs of different ages from the Thames 



at Twickenham, Middlesex. 



14. a-f. External aspect of a series of valves of the triangular 



form from the Cromerian (gravel) at West llunton, 

 Norfolk. 



15. a-Ti.~\ External aspect of a series showing variation, of the 



> rounded-triangular form from the Pleistocene at 



16. a-fr.J Grays, Essex. 



17. a-h. 1 of the elongate-triangular form from the same 



18. a-g. J deposits. 



PISIDIUM HENSLOWANUM, Shcppard. 



13. a-n. External aspect of a series of valves of all ages from the 



Pleistocene at Grays, Essex, for comparison with the 

 closely similar form of P. supinum shown at 17 & 18. 



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

 shows the amount of magnification. 



