PLATE XVIII— Con tinned. 



Productus vittatus, Hall. 



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

 Fig. 16. The interior of the bi-achial valve ; showing the muscular impressions 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, Iowa. 



Productus tenuioostatus, 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 jirofile ; showing the extremely produced anterior margin. 

 Keokuk limestone. New Providence, Indiana. 



