PLATE XI A— Continued. 



Derbya gkassa, Meek iiiid Haydeii. 



(See Plate X, tigs. 10, 11 ; and Plate Xlii, tigs. 23, 24.) 

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



Fig. 29. The cardinal process of the same specimen, enlarged ; showing the lohation at its summit ami the 

 extent of the crural plates. X 3. 



Upper Coal Measures. Kaiisas City, Missouri. 

 Fig. 30. The extei-ior of a larger pedicle-valve. 



Fig. 31. The intei-ior of a small peJicle-valve ; showing the median septum ami 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 intei-ior of a large bi-achial valve which is referred to this species with some doubt. The 

 c^'nral plates are well produce<l, partially enclo.sing arboi'escent scars of the adductoi' muscles. 

 The valve is unusually convex foi- one of this species. 



Upper Coal Measures. Kansas City, Missouri. 

 For other illustrations, see Paleontology of Eastern Nebraska, iilate v. tig. 10, and itlate viii, 

 fig. 1. 



Derbya Bennetti, .sp. nov. 



Fig. 34. The exterior of a pedicle-valve which letains the scar ot attachment at the umbo. 



Fig. 3.0. 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 nnsu)iported dental ridges, the tips of the grooved cardinal process, and the extrav- 

 agant developuient of the median septum. 



Fig. 39. A similar section of the same specimen near the apex. Here the medi.-m seiitum is coalesced with 

 a solid callosity filling the apical poi'tion of the deltidial cavity. 

 Upper Coal Measures. Kansas City, Missouri. 



