PRATE Vc—Continued. 
Orthis arcuaria, sp. llOV. 
(See Supplement for description.) 
Figs. 20, 21. Two views of a silicified, partially exfoliated shell; showing somewhat the contour of 0. ele- 
gantnla, but having a more strongly biconvex form. 
Niagara group. Perry county, Tennessee. 
Orthis Wisbyensis, Lindstrom. 
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 cardinal process, crural plates and muscular 
scars. X 2. 
Upper Silurian limestone. Island of Gotland. 
Orthis subcarinata, Hall. 
Figs. 25, 26, 27, 28. Views 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 interior of the brachial valve ; showing the limits of the muscular area, the 
ovarian spaces and the main vascular trunks. 
Fig. 32. A similar internal cast of the opposite valve, preserving the ramifications of the vascular sinuses, 
and showing also the ovarian spaces and muscular scars. 
Fig. 33. Posterior view of the cardinal 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. Y. 
For further illustration, see Palaeontology of New York, Volume III, plate xii, figs. 7-21. 
Orthis perelegans, Hall. 
Fig. 34. The interior of a brachial valye ; showing the divergent ridges extending from the line of separa¬ 
tion between the anterior and posterior elements of the adductor muscle. 
Fig. 35. Posterior view of the cardinal area of a brachial valve. 
Lower Helderberg group. Near 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 group. Cattaraugus county, N. Y. 
For further illustration, see Palaeontology of New York, Volume IV, plate viii, figs. 3-8. 
Orthis superstes, sp. nov. 
(See Supplement for description.) 
Fig. 44. Profile ; showing the convexity of the shell. 
Fig. 45. A view of the same specimen from the brachial valve. 
Fig. 46. An internal cast of the pedicle-valve; showing traces of the vascular sinuses. 
Fig. 47. The interior of the opposite valve ; retaining similar radiating furrows about the muscular area. 
Chemung group. Howard, N. Y. 
