PLATE XI. 
(Figures 1-22 by R. P. Whitfield.) 
Legend. D. Deltidium, pedicle-valve. 
C. Deltidium, brachial valve, 
t. Teeth. 
d. Dental lamellae. 
b. Dental sockets. 
c. Crural plates. 
s. Median septum, 
j. Cardinal process, 
r. Diductor scars, 
a. Anterior adductors. 
a 7 . Posterior adductors. 
Genus DERBYA, Waagen. 
Derby a Keokuk, Hall. 
Figs. 1,2. An internal cast and gutta-percha impression of the central cardinal area, viewed from the bra¬ 
chial valve ; showing the character and extent of the muscular area, cardinal process, dental 
lamellse and median septum. 
Keokuk beds (Knobstone group). New Providence, Indiana. 
Fig. 3. An partial cast of the interior of a pedicle-valve ; showing the muscular scars and impression of 
the median septum. 
Keokuk beds. Keokuk, Iowa. 
Derbya (?) biloba, Hall.* 
Fig. 4. A cardinal view ; showing the ventricose brachial valve and short area of the pedicle-valve. X 2. 
Fig. 5. A view of the brachial valve ; showing the bilobed contour of the shell, obdurate outline, the short 
cardinal area and deltidium of the opposite valve. X 2. 
Coal Measures. Winterset, Iowa. 
STREPTORHYNCHUS, King. 
Page 267. 
Streptorhynchus Hallianus, Derby. 
Fig. 6. A view from the brachial valve of an entire shell. 
Fig. 7. A posterior view of the cardinal process ; showing the lobation of the posterior face, the elevated 
crural plates and the grooved deltidium. X 2. 
Fig. 8. An enlarged view of the same parts, from above; showing also the character of the adductor im¬ 
pressions. X 2. 
Fig. 9. The cardinal process of a larger individual; showing the grooving of the extremities of the lobes. 
Fig. 10. A profile of the same; showing the backward extension of the process and the great elevation of 
the crural plates. X 2. 
Fig. 11. A posterior view of the same. X 2. 
Fig. 12. The cardinal process of another specimen ; showing a greater elevation and deeper bifurcation. 
Fig. 13. The exterior of an unsymmetrical pedicle-valve ; slightly restored about the left ante-lateral 
margin, and showing the strongly alternating striae. 
Fig. 14. The interior of the same valve; showing the unsupported teeth, and the character of the muscu¬ 
lar scars. 
Fig. 15. The exterior of a pedicle-valve, having a plicated exterior similar to that of Meekella and Strep- 
torhynchus pectinifoi-mis, Davidson. The shell is unsymmetrical and the outline is partially 
restored. 
Fig. 16. The interior of the same specimen; showing the cardinal area, closed delthyrium, the projecting 
teeth, and the flabellate muscular impression. 
Fig. 17. A view looking into the umbonal cavity of the specimen represented in fig. 14; showing the 
absence of dental lamellse and median septum. 
Upper Carboniferous. Bomjardim and Itaituba, Province of Para, Brazil. 
* Report of the New York State Geologist for 1882 (pub. 1883), explanation of plate (xi) 41, figs. 4 and 5. 
