PLATE XVIII— Couliiiucd. 



Peoductus vittatus, Hall. 



Fig. 15. A pedicle-valve with a somewhat exfoliated surface, but retaining- the concentric growth-lines. 

 Fig. 16. The interior of the brachial valve ; showing the muscular impressions and the cardinal process, 



bifid (in its upjier surface. 

 Fig. 17. The posterior fare of the cardinal process of the valve represented in fig. 16 ; showing a thi-ee-fold 



lobation. X 3 



Keokuk group. Near Keokuk, Iowa. 



Productus tenuicostatus, Hall. 



Fig. 18. A profile view ; showing the fine surface striation and the expanded anterior margins. 

 St. Louis limestone. Milan, Illinois. 



Productus ovatus, Hall. 



Fig. 19. A profile ; showing the extremely produced anterior margin. 

 Keokuk limestone. New Providence, Indiana. 



