I'LATE XVa— Conlinuerl. 



Genus DAVIDSONIA, Bouchard. 



rase 301. 



Davidsonia Verneuiliana, Bouchard. 



Fig. 31. Profile of an attacheii specimen retaining tlie two valves. 



Figs. 32, 33. Interiors of the brachial and pedicle-valves respectively. After Davidso.n. 



Genus PLECTAMBONITES, Paxder. 



(See Plate XV.) 

 Page 295. 



Plectambonites plicatella, Ulrieh. 

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

 Fig. 35. One of the shells, enlarged. ITie surface exposed is the exterior, the concave or brachial valve 

 showing, fi-oni comjiression, the outline of the cardinal process. X !>. 

 Hudson River g-roiiji. Covrington, 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 transvei-se musculai- ridges enclosing' the impres- 

 sion of the adductor muscles. X 3. 



Lower Helderberg group. Perry county. Tennessee. 



Genus RAFINESQUINA, gen. nov. 



(See Plate VIII.) 

 Page 2SI. 



Rafinesquina Ulrichi, James. 



Figs. 37, 3S. The exterior of two mature pedicle-valves in wiiich the 'primitive condition of the pedicle- 

 passage as a tube or sheath is i-etained. This appeai-s to be a normal feature of matunty in 

 this species. X 2. 



Hudson River group. Covington, Kentucky. 



Rafinesquina unicostata. Meek and Woiliieii. 



Fig. 39. The interior of a brachial valve; showing the canlinal process, dental sockets and nuiscular area. 

 Hudson River group. Sprbig Valley. Minnesota. 



Genus LEPT.ENA, Dalman. 



Page 276. 



LEPT^ffiNA RHOMBOIDALIS, WilckcilS. 



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



Fig. 40. The centi-al cardinal portion of a mature pedicle-valve ; showing the position of the inner ojiening 



of the foramen. A bristle has been drawm through the foramen to show that the passage was 



open at maturity. X 3. 



Lowei- Helderberg group. Near Clarkvsille, iV. F. 

 Figs. 41. 42. Two views of the younge.st 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. Waldro7i. Indiana. 



LEPT.ffiNA RHOMBOIDALIS, Var. VENTRICOSA, Hall. 



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

 diiluctor seal's and the thickened central adductor impression. 

 Oriskany sandstone. Cumberland, Maryland. 



