PLATE Vc— Continued. 

 Orthis ARCUARIA, sp. IIOV. 



(See Supplement for description.) 

 Figs. 20, 21. Two views of a silicified, partially exfoliated .shell; showing somewhat the contour of 0. eh- 

 gantula, but having- a more strongly biconvex form. 

 Niagara group. Peiry county, Tmmessee. 



Orthis Wisbyensis, Linclstrom. 



Fig. 22. Profile of the pedicle-valve. 

 Fig. 23. The interior of the same valve. 



Fig. 24. The interior of the brachial valve ; showing the cai-ilinal process, crural plates and mnscular 

 scars. X 2. 



Upper Silui'ian limestone. Island of Gotland. 



Orthis subcarinata, Hall. 



Figs. 25, 26, 27, 28. Vievv^s of the exterior of a normal individual ; showing its form and contour. 



Fig. 29. The interior of a brachial valve. 



Fig. 30. The interior of a pedicle-valve. 



Fig. 31. A natural cast of the intei-ior of the bi-achial valve ; showing the limits of the muscular area, the 



ovarian spaces and the main vascular trunks. 

 Fig. 32. A similar intei-nal cast of the opposite valve, preserving the ramifications of the vascular sinuses, 



and showing also the ovarian spaces and muscular scai-s. 

 Fig. 33. Posterior view of the cai-dinal area of the brachial valve ; showing the subdivision of the cardinal 



process and the elevation of the crural plates. 



Lower Helderberg group. Near Clarksville, N. T. 

 For further illustration, see Paleontology of New York, Volume III, plate xii, figs. 7-21. 



Orthis perelegans, Hall. 



Fig. 34. The interior of a brachial \-al\e ; showing the divergent ridges extending from the line of separa- 

 tion between the anterior and postei-ioi- elements of the adductor muscle. 

 Fig. 35. Postei'ior view of the cardinal area of a brachial valve. 



Lower Helderberg group. Neai- Clarksville, N. Y. 

 For further illustration, see Palaeontology of New York, Volume III, plate xiii, figs. 4-12. 



Orthis lenticularis, Vanuxem (= 0. lentiformis, Hall). 



(See note on page 224.) 

 Figs. 36, 37. Two views of a small individual. 



Corniferous limestone. LeRoy, N. Y. 

 Figs. 38, 39, 40. Views of a larger, more biconvex specimen. 

 Fig. 41. The interior of a pedicle-valve ; showing the character of the muscular area. 



Corniferous limestone. Caledonia, N. Y. 



Orthis Leonensis, Hall. 



Figs. 42, 43. Interiors of the brachial and pedicle-valves taken from impressions of natural moulds. 

 Chemung grouji. Cattaraugus county, iV. Y. 

 For further illustration, see Palteontology of New York, Vclume IV, plate viii, figs. 3-S. 



Orthis superstes, sp. nov. 



(See Supplement for description.) 

 Fig. 44. Profile ; showing the coiivexity of the shell. 

 Fig. 45. A view of the same specimen from the brachial valve. 



Fi"-. 46. An intei-nal cast of the jiedicle-valve ; showing traces of the vascular sinuses. 

 Fig. 47. The interior of the opposite valve ; j-etaining similar radiating furi-ows about the muscular area. 

 Chemvmg group. Howard, N. Y. 



