PLATE XII. 



PISIDITJM ASTARTOIDES, Sandberger. 



1. a-o. Internal aspect of a series of valves of all ages showing 



variation in form of outline and strength of hinge, 

 from the Pleistocene at Swanscombe, Kent. 



2. a-d. of four adult valves from the Pleistocene at Grays, 



Essex. 



3. Diagrammatic enlargement of the Hinge. 



4. -7t. Internal aspect of a series of valves of different ages 



showing variation, from the Pleistocene at Cray- 

 ford- Erith, Kent. 



5. a-k. from the Pleistocene at Clacton, Essex. 



[o^r is a right valve having the double cardinals proper 

 to the left valve; whilst a. ///. and p. in. are reduced 

 in size. 5 fc, on the other hand, is a left valve having 

 the dentition proper to a right valve.] 



6. rt, b. two adult valves from the Cromerian at West 



llunton, Norfolk. 



7. a, b. the same two more highly magnified. 



8. a, b. two valves from the Cromerian (gravel) of the same 



place. 



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

 shows the amount of maffnUicatioB. 



