PLATE XIA—Continued- 
Fig, 11. An internal cast of the brachial valve. 
Fig. 12. The cardinal process, enlarged. The lobes are strongly defined, the crural plates short and 
slightly recurving where they join the bottom of the valve, producing a thickening of the den¬ 
tal sockets. X 3. 
Lower Helderberg group. Albany county, N. Y. 
Orthothetes prava, Hall. 
Fig. 13. The interior of a brachial valve retaining the cardinal process, flabellate muscular scars and traces 
of vascular sinuses. 
Upper Devonian. Hackberry Grove, Iowa. 
Orthothetes Chemungensis, Conrad. 
(See Plate X, fig. 9.) 
Fig. 14. The cardinal process, enlarged; showing upon its summit the edges of the lobes on the posterior 
face. The transverse grooves at the junction of the process with the crural plates are made to 
appear somewhat too abrupt. Frequently the lobes of the cardinal process are more deeply 
divided than in this specimen. X 2|. 
Hamilton group. Western New York. 
For further illustration, see Palaeontology of New York, Volume IV, plates ix and x. 
Orthothetes crenistria, (Phillips ?) Meek. 
Fig. 15. The cardinal process, enlarged ; showing the character of its inner surface and the extent of the 
crural plates. X 3. 
Waverly group. Sciotoville, Ohio. 
For other illustration, see Palaeontology of Ohio, Volume II, plate x, fig. 5. 
Orthothetes lens, White. 
Fig. 16. The exterior of a pedicle-valve. 
Fig. 17. The interior of the same valve ; showing area, deltidium, teeth and muscular impressions. 
Fig. 18. The exterior of a brachial valve. 
Fig. 19. The interior of the same valve; showing cardinal process, muscular impressions and ovarian 
markings. 
Fig. 20. The cardinal process of the same specimen, enlarged to show its structure. Its outer face is cov¬ 
ered by the deltidium, which conforms to the grooved surface of the lobes. The crural plates 
are strongly recurved at their bases about the dental sockets. X 3. 
Choteau limestone. Louisiana, Missouri. 
Fig. 21. A very young pedicle-valve which retains the perforation at the apex of the deltidium. X 8. 
Fig. 22. Cardinal view of the same specimen; showing the height of the area and the prominence of the 
deltidium. X 8. 
Choteau limestone. Pike county, Missouri. 
Genus DERBY A, Waagen. 
(See Plates X, XI and XIb.) 
Page 261. 
Derbya Broadheadi, sp. nov. 
Fig. 23. Cardinal view of an entire, mature individual; showing the rugose and somewhat irregular ped¬ 
icle-valve, the more symmetrical brachial valve bearing a median sinus. 
Fig. 24. Profile view of the same specimen. 
Upper Coal Measures. Kansas City, Missouri. 
Derbya ruginosa, sp. nov. 
Figs. 25, 26, 27. Three views of an internal cast in chert, retaining portions of the inner laminae of the 
shell. Both valves were of irregular growth, the brachial valve not having the median sinus 
of the preceding species. There are many points of similarity between this fossil and the 
Orthis senilis, Phillips (Streptorhynchus crenistria, var. senilis, Davidson), which has been shown 
by Derby to have a septum in the pedicle-valve. 
Keokuk limestone. New Providence, Indiana. 
