PLATE XV— CoDtinned. 



Stropheodonta (Pholidostrophia) nacrea, Hall. 



Figa. 20, 21, 22. -Three views giving the genei'al external characters of (he species. 



Fig. 23. The interior of a bi'achial 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. Westeim New York. 



Genus PLECTAMBONITES, Pander. 



(See Plate XVa.) 



Page 295. 



Plectambonites sericea, Sowerby. 



Fig. 25. View of the bi'achial valve; showing the cardinal area and delli<liMm of the opposite valve. 



Fig. 26. The exterior of a pedicle-valve in which the strife altei'nate in size. 



Fig. 27. The interior of the brachial valve ; showing the depressed visceral disk, the muscular scai-s and 



cardinal process. 

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

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

 coalescence with the ends of the crui'al plates. X 3. 

 Fig. 29. The inteidor of the pedicle-valve ; showing the divergent muscular scai-s. 

 Ti'enton horizon. Minei'al Point, Wisconsin. 



Genus LEPT^NISCA, Beecher. 



Page 300. 



Lept^nisca concava, Hall. 



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

 Fig. 31. The inteiior of the same specimen ; showing the deltidium, the strong dental lamellae and the 

 median septum. X 2. 



Lower Helderberg grouja. Near Clarksville, N. Y. 



Genus CHRISTIANIA, gen. nov. 



Page 29S. 



Christiania subquadrata, Hall. 



Fig. 32. The interioi' of a brachial valve ; showing the bilobed cardinal process and the quadi'uple ad- 

 ductor scar, divided by high, vertical musculai' walls. 



Fig. 33. The intei-ior of the pedicle-valve, with an open delthj'rium, and showing the muscular walls and 

 scars. 



Lower Helderberg horizon. Pei-ry cmmty, Tennessee. 



Genus PLECTAMBONITES, Pander. 



Page 295. 



Plectambonites Transversalis, Daliiian. 



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. 



