PLATE XVIII— Coiilinued. 



Productus 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 musculai- impi'essions and the cardinal process, 



bifid on its upper surface. 

 Fig. 17. The posterioi- face of the cardinal process of the valve represented in fig. 16 ; showing a three-fold 



lobation. X 3 



Keokuk group. Near Keokuk, Imoa. 



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. 



