PLATE XV— ContinneU. 



Stropheodonta (Pholidostrophia) nacrea, Hall. 



Figs. 20, 21. 22. .Three views giving the general external characters of Ihe species. 



Fig. 23. The interior of a brachial valve ; showing the crenulated hinge-margin, cardinal process and 



nins<:nlar inii)ressions. 

 Fig. 24. The inteiior of the pedicle-valve ; showing the nmscular impi'essions and partially closed delthy- 



rium. 



Hamilton group. Western New York. 



Genus PLECTAMBONITES. Pander. 



(See Plate XV.\.) 

 Page 295. 



Plectambonites sericea, Sowciby. 



View of the brachial valve ; showing the eai-dinal area and deltidiinn of the opposite valve. 

 The exterior of a pedicle-valve in which the striae alternate in size. 



The interior of the brachial valve ; showing the depresseil 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 abrupt 

 tei'mination of the latter. The process is simple, but on its pcjsterior face appears trilobed fi'om 

 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, Wiscoitsin. 



Genus LEPT^NISCA, Beecher. 



Page 300. 



Lept^nisca concava, HiiU. 



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

 Fig. 31. The interior of the same specimen ; showing the deltidiuni, 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, H;ill. 



Fig. 32. Tlie intei-ioi- of a bi-acliial valve; showing the bilobed cardinal process and the quadruple ad- 

 ductor scar, divided by high, vertical muscular walls. 



Fig. 33. The inteiior of the pedicle-valve, with an open delthyrium, and showing the muscular walls and 

 scars. 



Lower Helderberg horizon. Fei'ry comity. Tennessee. 



Genus PLECTAMBONITES, Pander. 



Page 295. 

 ipLECTAMBONITES TRANSVERSALIS, Dllllliail. 

 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 bi-achial valve ; showing the elongate adductor scai-s. X 2. 

 Niagara group. Lockport, N. Y. 



