PLATE XI A— Conlinucrl- 

 Fig-, 11. An internal cast of the brachial valve. 



Fig-. 12. The cardinal process, enlarged. The lobes are sti-ongly defined, the crui-al plates short and 

 slightly recurving where they join the bottom of the valve, ijroducing a thickening of the den- 

 tal sockets. X 3. 



Lower Heldei-bei-g gi-oup. Albany cmmty, N. Y. 



Orthothetes pkava, Hall. 



Fig. 13. The interioi' of a brachial valve retaining the cardinal process, flabellate muscular scai-s and traces 

 of vascular sinuses. 



Upper Devonian. Haekberry Gh'ove, Iowa. 



Orthothetes Chemungensis, Comail. 



(See Plate X, fig. 9.) 

 Fig. 14. The cai-dinal pi'ocess, enlai-ged ; shovving upon its summit the edges of the lobes on the postei'ior 

 face. The transverse gi-ooves at the junction of the pi-ocess with the crural plates are niatie to 

 appear somewhat too abrupt. Precjuently the lobes of the cardinal process ai-e 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. (Phiilip.sV) Meek. 



Fig. 15. The cardinal process, enlai'geil ; showing the character of its inner sui"face and the extent of the 

 crural plates. X 3. 



Waverly group. Sciotoville, Ohio. 

 For other illustration, see Paljeoutology of Ohio. Volume II, plate x, tig. 5. 



Orthothetes lens. White. 



The exterior of a pedicle-valve. 



The interior of the same valve ; showing area, deltidium, teeth and muscular impressions. 

 The exterior of a brachial valve. 



The inteiior of the same valve ; showing caVdinal process, muscular impressions and ovai-ian 

 markings. 



Fig. 20. The cardinal process of the same specimen, enlarged to show its stnicture. Its outer face is cov- 

 ered by the deltidium, which conforms to the grooved surface of the lobes. The crural plates 

 are sti'ongly 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 jirominence 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. 11 ov. 



Fig. 23. Cardinal \'iew 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. Kaiisas City, Missouri. 



Derbya ruginosa, sp. IIOV. 



Figs. 25, 21), 27. Three views of an internal cast in chei't, retaining portions of the innei- laminae of the 

 shell. Both valves were of in egular 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 (Streptorliynclius crenistria. var. senilis. Davidson), which has been shown 

 by Derby to have a septum in the pedicle-valve. 



Keokuk limestone. New Providence, Indiana. 



Fig. 



16, 



Fig. 



17. 



Fig. 



18, 



Fig. 



n. 



i 



