PLATE XV— Cootinned. 



Stropheodonta (Pholidostrophia) nacrea, Hall. 



¥\gs. 20, 21, 22. -Three views giving' the general extei-iial chaiacters of the species. 



Fig. 23. The inteiior of a bi iiehial valve ; showing the ci-enulateil hinge-margin, cardinal process and 



muscular impressions. 

 Fig. 24. The inteiior of the pedic'le-valve ; t^howingthe muscular impressions anil partially closed delthy- 



rium. 



Hamilton group. Western New York. 



Genus PLECTAMBONITES. Pander. 



(See Plate XVa.) 

 Page 295. 



Plectambonites sericea, Sowerby. 

 View of the brachial valve; showing the ear<linal area and deltidiiiin of the opposite valve. 

 The exterio)- of a pedicle-valve in which the stiiae alternate in size. 



The interior of the Vtrachial valve ; showing the depressed visceral disk, the muscular scars and 

 cardinal process. 



Fig. 28. An enlargement of the cardinal process ; showing its union with the crural plates and the abrupt 

 tennination of the latter. The process is simple, but on its posterior face appears trilobed from 

 coalescence with the ends of the crural plates. X 3. 

 Fig. 29. The interior of the pedicle-valve ; showing the divergent muscular scare. 

 Trenton horizon. Mineral Point, Wisconsin. 



Genus LEPT^NISCA, Beecher. 



Page 300. 



Leptjenisca concava, Hall. 



Fig. 30. The exterior of a pedicle-valve ; showing the scar at the beak, due to the attachment of the shell. 

 Fig. 31. The interior of the same specimen ; showing the deltidium, the strong dental lamellce and the 

 median .septum. X 2. 



Lower Helderlierg group. Near Clarksville, N. Y. 



Genus CHRISTIANIA, gen. nov. 



Page 298. 



Christiania subquadraxa, Hall. 



Fig. 32. The interioi- of a bi'acliial valve ; showing the liilobed cardinal process and the quadruple ad- 

 ductor scar, divided by high, vertical muscular walls. 



Fig. 33. The interior of the jiedicle-valve, with an open delthyrium, and showing the muscular w,alls and 

 scars. 



Lower Helderberg horizon. Perry county, Tennessee. 



Genus PLECTAMBONITES, Pander. 



I*age 295. 



Plectambonites tran.'^versalis, Dalmaii. 

 Fig. 34. The exterior of a pedicle-valve. X 2. 

 Fig. 35. A view from the opposite valve. X 2. 



Fig. 36. The interioi- of a brachial valve ; showing the elongate adductor scars. X 2. 

 Niagara group. Ijockport, N. Y. 



