PLATE XI B. 



(Fisfures 2-17, 20-i4 by (i. B. biJU-soN; 1, W, 19 by E. Emmons.) 

 Leg-end. i. Cardinal jirocess. ' i-v. Rostral vault, 



(1. Dental laraellae. dr. Dental Hdges, 



t. Teeth. 



Genus DERBYA, Waaqen. 



Page -261. 



(See Plates X, XI and XIa.) 



Derbya, sp. indet. 



Fig. 1. The interior of a large and very convex brachial valve ; showing the cai-dinal process and short, 

 vertical crural plates, from the base of which a thickened ridge extends about the muscular 

 area. The accompanying outline profile shows the convexity of the valve. 

 Uppei' Coal Measures. Kaiisas City, Missouri. 



Derbya cymbula, sp. nov. 



Fig. 2. Cai'diiial view of a large iudiviilual, somewhat incomplete at the apex ; showing the height of the 

 area, the median gi'oove on the surface of the deltidium, and the tendency to irregular growth 

 in the pedicle-valve. The broken suiface at the apex of the pedicle-valve shows evidence of 

 the existence of a median septum. 



Fig. 3. View of the brachial valve of the same specimen ; showing its form, contour and the character of 

 the surface ornamentation. 



Figs. 4, .5. Cardinal and profile views of a small individual, probably belonging to the same species. In 

 this sjiecimen the iiregular growth is very pronounced in both valves. 

 Upper Coal Measures. Near Kansas City, Missouri. 



Derbya Kaskaskiexsis, Mc( 'licsiicy. 



Fig. tJ. The interior of an imperlecl pedicle-valve, in which the median septum unites with the dental 

 lamellse, forming an elongate deltidial or rostral vault, like that seen in Derbya Correana, Deil>y 

 (plate xi, tig. 22). 



Chester limestone. Crifte7iden county. Kentucky. 



Derbya robusta (?), Hull. 



(See Plate X, figs. 12-17.) 

 Fig. 7. The interior of a brachial valve of an old shell, which probably belongs to this species. The 

 cardinal jirocess is much thickened and slightly irregular in its growth, the muscular area deeply 

 excavated but not well defined, and the pallial region covered with traces of the vascular sinu- 

 ses. The outline at the left shows the convexity of the valve. 

 Upper Coal Measures. Winterset, Iowa. 

 Fig. S. The inteiior of a pedicle-valve, having a comparatively narrow caidinal area, a relatively small 

 flabellate muscular impression, and a short median septum. 

 Upper Coal Measures. Near Kansas City, Missouri. 



Genus STKKPTORnYNCHL'S. Kino. 



I'aijc -^67. 



Streptorhynchus pelargonatus. Sclllothcilll. 



Fig. 9. An internal cast of the two valves; showing the position of the deltidium and dental ridges of 

 the pedicle-valve, the inner face of the cardinal process, and the muscular impressions of the 

 brachial valve. X 'S. 



Figs. 10, 11, 12. Anterior, profile and cardinal views of a specimen which retains the shell ; showing the 

 contour and general external charactei-s of the species. X 2. 



Fig. 13. An internal ca.st of the pedicle-valve; showing the form of the nuisculai' area, without evidence 

 of a median septum. X 2. 



Fijr. 14. The interior of the cardinal portion of the articulated valves ; showing the dental ridges and imier 

 surface of the deltidium, the cardinal process, crural plates and the character of the articula- 

 tion. X 5. 



Permian Formation. Posneck, Silesia. 

 For further illustration, see Gbinitz, Dya.s, plate xvi, figs. 26-34 ; Davidson, Permian Brachio)>oda, 

 plate ii, figs. 32-42. 



