PLATE XVa— Continued. 



Genus DAVIDSONIA, Bouchard. 



Page 301. 



Davidsonia Verneuiliana, Bouchard. 

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

 Pigs. 32, 33. Interiors of the brachial and pedicle-valves respectively. After Davidsoit. 



Genus PLECTAMBONITES, Pander. 



(See Plate XV.) 



Page 295. 



PLECTAMBONITES PLIOATELLA, Ull'ich. 



Fig. 84. A group of these shells which have fallen into the sediment with their valves attached and open. 

 Fig. 35. One of the shells, enlai-ged. The surface exposed is the extei'ior, the concave or brachial valve 

 showing, from compression, the outline of the cardinal process. X T). 

 Hudson River group. Covingto7i, Kentucky. 



Genus CHRISTIANIA, gen. nov. 



Page 298. 



Cheistiania subquadrata, Hall. 



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

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

 crui'al plates, the prominent long'itudinal and transvei'se muscular ridges enclosing the impres- 

 sion of the adductor muscles. X 3. 



Lower Helderberg group. Peiry county, Tennessee. 



Genus RAFINESQUINA, gen. nov. 



(See Plate VIII.) 

 Page 2S1. 



Eafinesquina Ulrichi, James. 



Figs. 37, 38. The exterioi' of two mature pedicle-valves in which fhe/pi-imitive condition of the pedicle- 

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

 this species. X 2. 



iTudson River group. Covington, Kentucky. 



Rafinesquina unicostata, Meek aud 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 LEPT^NA, Dalman. 



Page 276. 



LEPT.ffiNA RHOMBOiDALis, Wilckens. 

 (See Plate VIII, tigs. 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 jiassage was 

 open at maturity. X 3. 



Lower Helderberg group. 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 obseiwed condition. X 10. After Beecher and Clarke. 

 Niagara group. Waldron, Indiana. 



LePT^ENA RHOMBOIDALIS, VaV. VENTRICOSA, Hall. 



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

 didiictor scars and the thickened central adductor impression. 



Oriskany sandstone. Cumbei-land, Maryland. 



