PLATE XV—Continued. 
Stropheodonta (Pholidostrophia) nacrea, Hall. 
Figs. 20, 21, 22. 'Three views giving the general external characters of the species. 
Fig. 23. The interior of a brachial valve; showing the crenulated hinge-margin, cardinal process and 
muscular impressions. 
Fig. 24. The interior of the pedicle-valve ; showing the muscular impressions and partially closed delthy- 
rium. 
Hamilton group. Western New York. 
Genus PLECTAMBONITES, Pander. 
(See Plate XVa.) 
Page 295. 
Plectambonites sericea, Sowerby. 
Fig. 25. View of the brachial valve ; showing the cardinal area and deltidium of the opposite valve. 
Fig. 26. The exterior of a pedicle-valve in which the striae alternate in size. 
Fig. 27. The interior of the brachial 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 
termination 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 scars. 
Trenton horizon. Mineral Point, Wisconsin. 
Genus LEPTiENISCA, Beecher. 
Page 300. 
Leptjenisca concava, Hall. 
Fig. 30. The exterior of a pedicle-valve j 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 lamellae and the 
median septum. X 2. 
Lower Helderberg group. Near Clarksville, N. Y. 
Genus CHRISTIANIA, gen. nov. 
Page 298. 
Christiania subquadrata, Hall. 
Fig. 32. The interior of a brachial valve; showing the bilobed cardinal process and the quadruple ad¬ 
ductor scar, divided by high, vertical muscular walls. 
Fig. 33. The interior of the pedicle-valve, with an open delthyrium, and showing the muscular walls and 
scars. 
Lower Helderberg horizon. Perry county, Tennessee. 
Genus PLECTAMBONITES, Pander. 
Page 295. 
Plectambonites transversalis, Dalman. 
Fig. 34. The exterior of a pedicle-valve. X 2. 
Fig. 35. A view from the opposite valve. X 2. 
Fig. 36. The interior of a brachial valve ; showing the elongate adductor scars. X 2. 
Niagara group. Lockport, N. Y. 
