PLATE XV A— Continued. 



Genus DAVIDSONIA, Bouchard. 



I';ige 301. 



Davidsonia Verneuiliana, Boucliiud. 



Fig. 31. Profile of an attached specimen retaining the two valves. 



Figs. 33, 33. Interiors of the brachial and pedicle-valves I'espectively. After Davidson. 



Genus PLECTAMBONITES, Pander. 



(See Plate XV.) 

 Page 295. 



Plectambonites plicatella, Ulrich. 



Fig. 34. A group of these shells which have fallen into the sediment with their valves attached and open. 

 Kig. 35. One of the shells, enlarged. The sui-face exposed is the exterior, the concave oi- brachial valve 

 showing, from comps'ession, the outline of the cardinal process. X •'■ 

 Hudson River grouji. Coiimgton, Kentucky. 



Genus CHRISTIANIA, gen. nov. 



Page 298. 

 Christiania SUBQUADRATA, HilU. 



(See Plate XV, figs. 32, 33.) 

 Fig. 36. Cardinal view of the lirachial valve ; showing the bilobed cardinal process, widely divergent 

 crural plates, the prominent longitudinal and transvei'se muscidar ridges enclosing the impi'es- 

 sion of the adductor muscles. X 3. 



Lower Helderberg group. Perry county, Te7ine,i.iee. 



Genus RAFINESQUINA, gen. nov. 



(See Plate VIII.) 

 Page 281. 



Rafinesquina Ulrichi, James. 



Figs. 37, 38. The exterior of two mature )iedicle-valves in which the 'i>rimitive condition of the pedicle- 

 passage as a tube oi' sheath is retained. This appears to be a. nonnal feature of maturity in 

 this species. X 2. 



Hudson River group. Covington, Kentucky. 



Rafinesquina unicostata, Meek and Wortheii. 



Fig. 39. The interior of a brachial valve; showing the cardinal process, dental sockets and musculai" area. 

 Hudson River group. Spring Valley, Minnesota. 



Genus LEPT^NA, Dalman. 



Page 276. 



Leptj^na rhomboidahs, Wilckeus. 



(See Plate VIII, figs. 17-31 ; Plate XX, figs. 21-2.').) 

 Fig. 40. The centi-«l cardinal portion of a mature pedicle-valve ; showing the position of the inner opening 

 of the foramen. A bi-istle has been di-awn through the foi'amen to show that the passage was 

 open at maturity. X 3. 



Lower Helderberg group. Near Clarkvsille, iV. Y. 

 Figs. 41, 42. Two views of the youngest specimen obtained; much enlarged to show the structure of the 

 pedicle-passage in its earliest observed condition. X 10. After Beeoher and Clarke. 

 Niagara group. JValdron, Indiana. 



Lept.^na rhomboidalis, var. ventricosa, Hall. 



Fig. 43. The interior of a pedicle-valve ; showing the gi-eat prominence of the muscular ridges about the 

 diductoi- sc:ars and the thickened central adductor impression. 

 Oriskany sandstone. Cumberland, Maryland. 



