PLATE XI a—C ontinued. 
Derby a crassa, Meek and Hayden. 
(See Plate X, tigs. 10, 11; and Plate XIb, figs. 23, 24.) 
Fig. '28. The interior of a small brachial valve. 
Fig. 29. The cardinal process of the.same specimen, enlarged ; showing the lobation at its summit and the 
extent of the crural plates. X 3. 
Upper Coal Measures. Kansas City, Missouri. 
Fig. 30. The exterior of a larger pedicle-valve. 
Fig. 31. The interior of a small pedicle-valve; showing the median septum and large muscular scars. 
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 large brachial valve which is referred to this species with some doubt. The 
crural plates are well produced, partially enclosing arborescent scars of the adductor muscles. 
The valve is unusually convex for one of this species. 
Upper Coal Measures. Kansas City, Missouri. 
For other illustrations, see Palseontology of Eastern Nebraska, plate v, fig. 10, and plate viii, 
fig. 1. 
Derbya Bennetti, sp. 110V. 
Fig. 34. The exterior of a pedicle-valve which retains 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. 36. 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 having the pedicle-valve cut horizontally near the hinge. The section 
shows the unsupported dental ridges, the tips of the grooved cardinal process, and the extrav¬ 
agant development of the median septum. 
Fig. 39. 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. 
Upper Coal Measures. Kansas City, Missouri. 
