PLATK V— Continued 

 Vig. 19. The deltulial portion of the pedicle-valve ; showing the liioad delthyrium and the outline of the 



musculai- area which is not sufficiently thickened to obscure the plications of the shell. X 3- 

 V\g. 20. The area in the opposite valve : showing the cardinal process, crural [ilates and low median 

 ridge. X 3- 



Hud.son River gi-oup. Covhii/ton, Kentucky. 

 For further illustration, see PaL-Eontology of Ohio, Volume I. plate viii, tig. 7. 



Orthis dichotoma, Hall. 

 O. ilirlidtdiiKi, Hall, r= 0. fssicosta. Meek, not Hall. 



Fig. '21. The interior of the br;ichial valve ; showing the cardinal process, crural plates and the charactei' 

 of the plications. 



Hudson River group. Covingtm, Kentuvhy. 

 For further illu.stration, see Palaeontology of Ohio, Volmne I, plate viii, lig. i'>, under the name of 

 0. jbi.i'i casta. 



Orthis Ella, Hull. 



Fig. 2'J. An inilividual viewed tVom the lu-achial valve, showing area and foramen of the opposite 



valve. X 2. 

 Fig. 23. A poi'tion of the interior of the pedicle-valve ; showing the characters of the cardinal ai-ea. X 3. 

 Hudson River group. Covington, Kentucky. 

 For further illustration, see Palaeontology of Ohio, Volume I, plate viii, fig. 9. 



Oiiiiiis K.VXKAKENSIS, McChcsiicy. 



Figs. 24, 25. Brachial and jirutile views of a n<irinal individual. 



Hudson River group. XViliiihigUm, Ulhiois. 



Orthis Whitfieldi, N. U. Wincholl. 



Fig, 26. The inteiior of a pedicle-valve ; showing the relatively high area and .strong teeth, the character 

 of the muscular area, and the low ridges radiating fi'om its anterior margin. 

 Hudson River group. Kpring Valley, Minne.'^oto. 



DINORTPUS, NOM. NOV. 



Page. 195. 



Orthis pectinella (Einnion.s), Coinad. 



Figs. 27, 28. A view of the pedicle-valve, and protile of a .specimen in which the I'ibs are more subdivided 

 toward the margin than is usual in the species. X 2. 

 Trenton limestone. Lewis county. New York. 

 Fig. 29. The interior of a small pedicle-valve; showing the subqiuidrate muscular area with the posterior 

 .and anterioi' diductor and the .adductor scars. 

 Trenton horizon. f!t. Paul, Minnesota. 

 Fig.s. 30, 3L A view from the ]iedicle-valve and the protile of a large individual, sluiwiug the .strongly ele- 

 vated, siinjile ribs. 

 Fig. 32. The interior of a pedicle-valve. 

 Fig. 3'!. The cardinal portion of a brachial valve. X 2. 



Ti'enton hoiizon. Curdsville, Kentucky. 

 For further illustration, .see Palaeontology of New Y(n'k, Volume I, jilate xxxii, tig. ]', 



Orthis Sweeneyi, N. H. Wimholl. 



Fig. 34. Exterior of the pedicle-valve ; showing the depre.s.scd-convex shell. 



Fig. 3."). A jii-fifile of conjoined valves; in this lignre the pedicle-valve as I'epresented is too convex and too 



higli at the head. 

 Fig. 30. The interior of the brachial valve. 



Trenton horizon Ht. Paul, Minne.'iota. 



ORTHIS as rcsfiictcd. 



Page 1U2. 



Orthis flabellites, Hall. 



Figs. 37, 38, 39. The exterior of the pedicle and brachial valves and profile of an average specimen. 

 Fig. 40. The interioi- of the pedicle-valve ; showing the character of the uuiscular area. 

 Fig. 41. The interior of the brachial valve ; showing the cardinal process, crural plates ami cpiadripartite 

 muscular area. 



Niagara group. Lockport, N. Y. 

 F'or further illustraticm see Palaeontology of New York, VolimK; II, ]ilate lii, figs. 6, 7. 



