rLAFK XI A-Coiililiucil- 

 Fig 11. An internal cast of the, In-achial valve. 



Fig-. 12. The oanlinal iirocess, enlarf<-e<l. The lobes ave sti-ongly detineil, the crural plates short and 

 sliffhtly recui-ving where they join the bottom of the valve, producing a thickening of the den- 

 tal sockets. X 3. 



Lower Helderberg group. Albany comity. N. Y. 



OUTHOTHETES PKAVA, Htlll. 



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

 of vascular sinuses. 



Upper Devonian. Hackberry Cf-rove, Iowa. 



OuTHOTHETES CnEMUNGENSIS, C'olllMil. 

 (See Plate X, tig. 9.) 

 Fig. U. 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 piocess with the ci-ural plates are made to 

 appear somewhat too abinijit. Frequently the lobes of the cardinal pi'ocess are moi'e deeply 

 divided than in this specimen. X 2^. 



Hamilton group. We.itern. New York. 

 For further illustration, see Palseontology of New York, Voliune IV, plates ix and x. 



Okthothetes CKENisTUiA, (Phillip.-^ ?) Meek. 



Fig. l."). The cardinal process, enlarged ; showing the character of its inner surface and the extent of the 

 cnnal plates. X 3. 



Waverly group. SciotovUle, Ohio. 

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



OrTHOTHETES LENS, AVliito. 

 Fig. 11). The exterior of a pedicle-valve. 



Fig. 17. U'he interior of the same valve ; showing are.a, <leltidium, teeth and muscular imjiressions. 

 Fig. 18. The exterior of a brachial valve. 



Fig. Hi. The interior of the .«ame valve; showing cai-dinal process, muscular iinpressicms and ovarian 

 markings. 



Fig. 20. The cardinal process of the same specimen, enlai'ge<l to show its structure. Its outer face is cov- 

 ered l)y the deltidium, which conforms to the grooved surface of the lobes. The ci'ural plates 

 are strongly recurved at their liases about the dental sockets. X 3. 

 Choteau limestone. Louisiana, Missouri. 



Fig. 21. A very young jiedicle-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.) 

 P.iRe 261. 



Derbya Broadheadi. sp. IIOV. 



Fig. ^3. Carduial view of an entire, mature indivi<bial ; showing the rugose and somewhat irregular jied- 



icle-valve, the more .symmetrical brachial valve bearing a median sinus. 

 Fig. 24. Protile view of the sanje specimen. 



Upper Coal Measures. Ka7i.ia.i City. Mis.'smiri. 



Derbya rugikosa, sp. nov. 

 Figs. 25, 2t;, 27. 'ITiree views of an internal east in chert, retaining portions of the inner laminae of the 

 shell. Both valves wei^e of iriegular growth, the br.'u:hial valve not having the median sinus 

 of the preceding species. Thei-e ai-e many points iif similarity between this fossil and the 

 Orthis .senilis, Philliiis (Streptorliyiichm creni.sMa. var. senilis. Davidson), which has been shown 

 liy Dkbbv to have a septum in the jiedicle-valve. 



Keokuk limestone. New Providence, Indiana. 



