PLATE XXX. 



PISIDITTM OBTUSALE, (Lamarck) Jenyns. 



1. a-h. External right aspect of a series of pairs of different 



ages showing variation, from Birch, Essex. 



2. a, b. 1 Posterior and dorsal view of two pairs from the same 



3. a, b. J locality. 



4. a, 6. of a pair from Guildford, Surrey. 



5. a, b. of a pair from Swinton, Lancashire. ("P.nitidum, 



var. ylobosa, Jeffr." : T. llogers coll.) 



6. a, b. of a pair from OvergUde, Mdlselvdalen, Norway. 



(" P. scholtzi, Clessin.") 



7. a, b. Exterior and 'interior right view of an abnormal pair 



from Walworth Wood, Co. Londonderry. 



8. a-c. External right aspect of three pairs showing variation in 



form, from Lochmaben, Dumfriesshire. 

 0. a, b. right and left aspect of a pair from Llandaff, 



Glamorganshire. 



10. External right aspect of a pair from Guildford, Surrey. 



1J.. Diagrammatic enlargement of the Hinge. 



J2, a, b. Internal aspect of a pair from Guildford, Surrey. 



1.3. , b. of a pair from Llandaff, Glamorgan. 



1.4. rt, b. of a pair from Iteydon Marsh, Suffolk. 



15. a, b. of a pair from Lytham, Lancashire. 



16. o-o. of a series of different ages showing variation, from 



Birch, Essex. 



17. a, b. of a pair from Ovcrgade, Malselvdalen, Norway. 



(" P. scholtzi, Clessin.") 



18. a, b. of a pair from Lochmabeu, Dumfriesshire. 



19. of a right valve from Swinton, Lancashire. 



("P. niliclum, vaT.globosa, Jeffr." : T. Rogers coll.) 



20. , b. of a pair from Criccieth, Carnarvonshire. 



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

 shows the amount of magnification. 



