ri-ATK Vc— Coiilinncd. 

 OUTHIS AKCUARIA, sp. IIOV. 

 (See Supplement foi' description.) 

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

 gantula, but having a more strongly biconvex form. 

 Niagara group. Perry county, Tennessee. 



Orthis Wisbtensls, Lindstiom. 



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 jilates and muscular 

 scars. X 2. 



Upper Siluiian limestone. Island of Gotland. 



Orthis subcarinata. Hall. 



Fi"^. 25, 26, 27, 28. Views of the exterior of a noi'uial individual ; showing its form and contour. 



Fig. 29. The interior of a brachial valve. 



Pig. 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 \ascular trunks. 

 Fig. 32. A similar internal cast of the opposite valve, preserving the ramifications of the vascidar 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 cardina' 



process and the elevation of the crural plates. 



Lower Helderberg group. Near Clarksville, iV. Y. 

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



Orttiis perelegans, Hall. 



Fig. 34. The intei'ior of a Ijr.achial valve ; showing the ilivei'geiit ridges extending from the line of separa- 

 tion between the anterior and posterior elements of the aciductor muscle. 

 Fig. 35. Posterior view of the cardinal ai-ea of a brachial valve. 



Lower Helderberg gi-iiui). Near Clarksville, N. Y. 

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



Orthis lenticularis, Vanuxem (= 0. lentifonvi.^. Hall). 

 {See note on page 224.) 

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



Corniferous limestone. Lelioy, N. 1'. 

 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, jSf. Y. 



Orthis Leonensis, Hall. 



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

 Chemung group. Cattaraugus county, N. Y. 

 For further illustration, see Paleontology of New Yoik, Vc lume IV, plate viii, figs. 3-8. 



Orthis superstes, sp. iiov. 



(^ee 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 vivscnlar sinuses. 

 Fig. 47. The intenor of the opposite valve ; retaining similar radiating furrows about the muscular area. 

 Chemung group. Hofward, N. Y. 



