PLATE XVI. 



PlSIDITTM CA.SERTAXUM (Poll). 



1. af. Internal aspect of a series of three pairs from Hendon, 



London. 



2. a-f. from Damage Pond, Mortehoe, Devon. 



3. a~d. of two pairs from Borough Stream, Mortehoe, Devon. 



4. a-c. of a pair and an odd valve from Lochmaben, Dum- 



friesshire. 



5. a-7. of a series of pairs of the lake form, from Bassen- 



thwaite, Cumberland. 



6. a-c. of two pairs from Shirwell, Devon. 



1 . a, b. of an abnormal pair from Newtownards, Co. Down. 



8. a, b. of the Hinge of the same enlarged. 



[The right valve shows a double, instead of a 

 single, cardinal ; whilst a. in. is suppressed and 

 p. in. reduced. In the left valve the lateral 

 teeth are doubled as in a normal right valve.] 



9. a-d. of two pairs from Vestre Aker, Christiania. 



(" P. globulare, Clessin.") 



10. o, b. of a pair from Mt. Majella, Italy. ("P. casertanum , 



var. lenticularis, Norm." : R. Bellini.) 



11. , b. of a pair from Lancaster. 



12. a, b. of a pair from Bennett's Mouth, Mortehoe, Devon. 



13. a, b. of a pair from Coolkeeragh, Co. Londonderry. 



14. a, b. of a pair from Lagan Canal (2nd lock), Cos. Down 



and Antrim. 



15. a, b. of a pair from Wniana, Roumania. (" P. fossari- 



num, Clessin.") 



16. , b. of a pair from the Sebeto, Naples. 



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

 shows the amount of magnification. 



