PLATE XVa—C ontinued. 
Genus DAVIDSONIA, Bouchabd. 
Page 301. 
Davidsonia Verneuiliana, Bouchard. 
Fig. 31. Profile of an attached specimen retaining the two valves. 
Figs. 32, 33. Interiors of the brachial and pedicle-valves respectively. After Davidson. 
Genus PLECTAMBONITES, Pander. 
(See Plate XV.) 
Page 295. 
PLECTAMBONITES PLICATELLA, Ulfich. 
Fig. 34. A group of these shells which have fallen into the sediment with their valves attached and open. 
Fig. 35. One of the shells, enlarged. The surface exposed is the exterior, the concave or brachial valve 
showing, from compression, the outline of the cardinal process. X 5. 
Hudson River group. Covington, Kentucky. 
Genus CHRISTIANIA, gen. nov. 
Page 298. 
Christiania subquadrata, Hall. 
(See Plate XV, figs. 32, 33.) 
Fig. 36. Cardinal view of the brachial valve; showing the bilobed cardinal process, widely divergent 
crural plates, the prominent longitudinal and transverse muscular ridges enclosing the impres¬ 
sion of the adductor muscles. -X-3 — y^Lj- 
Lower Helderberg group. Perry county, Tennessee. , ( 
Genus RAFINESQUINA, gen. nov. 
(See Plate VIII.) 
Page 281. 
Rafinesquina Ulrichi, James. 
Figs. 37, 38. The exterior of two mature pedicle-valves in which the '’primitive condition of the pedicle- 
passage as a tube or sheath is retained. This appears to be a normal feature of maturity in 
this species. X 2. 
Hudson River group. Covington, Kentucky. 
Rafinesquina unicostata, Meek and Worthen. 
Fig. 39. The interior of a brachial valve; showing the cardinal process, dental sockets and muscular area. 
Hudson River group. Spring Valley, Minnesota. 
Genus LEPTiENA, Dalman. 
Page 276. 
Lept.®na rhomboidalis, Wilckens. 
(See Plate VIII, figs. 17-31; Plate XX, figs. 21-25.) 
Fig. 40. The central cardinal portion of a mature pedicle-valve ; showing the position of the inner opening 
of the foramen. A bristle has been drawn through the foramen to show that the passage was 
open at maturity. X 3. 
Lower Helderberg gitfiup. Near Clarkvsille, N. T. 
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 Beecher and Clarke. 
Niagara group. Waldron, Indiana. 
Lept^na rhomboidalis, var. ventricosa, Hall. 
Fig. 43. The interior of a pedicle-valve ; showing the great prominence of the muscular ridges about the 
diductor scars and the thickened central adductor impression. 
Oriskany sandstone. Cumberland, Maryland. 
