PLATE XVIII—Con tinued. 
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 muscular impressions and the cardinal process, 
bifid on its upper surface. 
Fig. 17. The posterior 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 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. 
