PLATE XII. 



PrsioiUM ASTARTOIDES, Sundberger. 



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. a-Ji. Internal aspect of a series of valves of different ages 



showing variation, from the Pleistocene at Cray- 

 ford-Erith, Kent. 



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



[5 cj is a right valve having the double cardinals proper 

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

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

 the dentition proper to a right valve.] 



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



Bun ton, Norfolk. 



7. a, 6. the same two more highly magnified. 



8. , 6. 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 magnification. 



