PLATE XI A— Continued. 



Derbya crassa, Meek and Hayden. 



(See Plate X, figs. 10, 11 ; iind Plate XIb, fig-s. 23, 24.) 

 Fig. 28. The interior of a small brachial valve. 



Fig. 29. The cardinal process of the same specimen, enlai'ged ; showing the lobation at its summit and the 

 extent of the crural plates. X 3. 



Upper Coal Measui'es. Kansas City, Missouri. 

 Fig. 30. The exterior of a larger pedicle-valve. 



Fig. 81. The intei-ior of a small pedicle-valve ; showing the median septum and large muscular scai-s. 

 Fig. 32. The interior of a large and old pedicle-valve ; showing the median septum, the thickening about 

 the muscular area, due to age, and marks of vascular sinuses. 

 Upper Coal Measures. Winterset, Iowa. 

 Fig. 33. The interior of a lai'ge brachial valve which is referred to this species with some doubt. The 

 crural plates are well prodticed, partially enclosing arborescent scars of the adductoi' muscles. 

 The valve is unusually convex for one of this species. 



Upper Coal Measures. Kansas City, Missouri. 



For other illustrations, see Palaeontology of Eastern Nebraska, plate v, fig. 10, and plate viii, 

 fig. 1. 



Derbya Bennetti, sji. nov. 



Fig. 34. The extei'ior of a pedicle-valve which letains the scar of attachment at the umbo. 



Fig. 35. The opposite side of the same specimen ; showing the height of the cardinal area of the pedicle- 

 valve and the median sinus on the brachial valve. A Crania is attached to the apex of the 

 latter valve. 



Fig. 86. Profile view of another specimen. 



Fig. 37. Cardinal view of the same ; showing the high cardinal area and the lack of symmetry in the ped- 

 icle-valve. 



Fig. 38. The same specimen after hajving the pedicle-valve cut horizontally near the hinge. The section 

 shows the unsuppoi-ted dental ridges, the tips of the grooved cardinal pi-ocess, and the extrav- 

 agant development of the median sei^tum. 



Fig. 89. A similar section of the same specimen near the apex. Here the median septum is coalesced with 

 a solid callosity filling the apical portion of the deltidial cavity. 

 Uppei' Coal Measures. Kansas City, MissouH. 



