PLATE VlA-Continueii. 



Okthis Thiemii, Wliite. 



Pig-. 14. The interior of the pedicle-valve. X 3. 



Fig. 15. The interior of a brachial valve which retains the bases of the crura at the extremities of the crural 

 plates. X H. 



Burlington limestone (arenaceous Ijeds). Burlinc/ton, Iowa. 

 The specimen referred to thi.s species, with doubt, in Palaeontology of New York, Volume IV, p. 

 63, plate viii, tig. 2, is not 0. T/dmnli, and will requii'e a diffei-ent reference. 



Orthis Missouuiensis,* Swallow. 

 Figs. 16, 17. Intei-iors of the brachial and i^edicle-valves. The asymmetry of the muscular area is an ac- 

 cidental misrejaresentation. 



Choteau limestone. Pike county, Missoun. 



Orthis dubia, Hall. 



Figs. 18, 22. Views of oppo.site sides of the same specimen ; showing the peculiar form of the shell. X 2. 



St. Louis gi'oujj. Lebanon, Kentucky. 

 Fig. 19. The interior of a pedicle-valve ; showing the large delthyrium and strong teeth with the absence 

 of a cardinal area. X 2. 

 [The limitations of the muscular area are shown, but its subilivisions are not preserved. 

 St. Louis group. Lancsville, Indiana. 

 Fig. 20. Interior of the pedicle-valve, retaining more distinctly the subdivisions of the muscular area. X 2. 



Chestei' limestone. Litchflekl, Kentucky. 

 Fig. 21. The interior of a bi-achial valve ; showing the greatly thickened and elevated hinge-plate bearing 

 the cai'dinal process and crural plates. X 2. 

 St. Louis groujj. LaucwiUe, Indiana. 



SCHIZOPHORIA, King. 



P.ige '211. 



Orthis senectus, sp. nov. 



Fig. 23. The exterior of a pedicle-valve; showing its depression over the pallial region. 

 Fig. 24. An internal cast of the pedicle-valve ; showing the diductoi' and adductor scai-s. 

 Clinton group. Heynale'.^ Bajsin, N. Y. 



Orthis multlstriata, Hall. 



Fig. 2.5. An internal cast of the bi-achial valve of one of the original specimens ; showing the subdivision 

 of the muscular ai'ea and the di\'erging vascular sinuses. 



Lower Heldei-berg group. Near Clarksville, N. Y. 

 For further illustration, see Palfeontology of New York, Volume III, plate xv, fig. 2. 



Orthis impressa, Hall. 



(See Plate VI, fig. 31.) 

 Figs. 26, 27. Two internal casts of the brachial valve, which retain with unusual distinctness the muscular, 

 vascular and ovarian markings, and also the impression of the multipartite cardinal process. 

 Chemung group. Lawrenceville, Pemisylvania. 

 For farther illustration, see Palaeontology of New York, Volume IV, plate viii, figs. 11-19. 



Orthis Iowensis, Hall. 



Fig. 29. The interloi- of a pedicle-valve ; showing the musculai- area with a strongly thickened and elevated 

 central adductor impression. 



Chemung group. Lime Creek, Iowa. 

 See Geology of Iowa, Volume I, part ii, plate ii, figs. 4 a-i. 1858. 



Orthis Macfarlanii, Meek.f 



Fig. 28. The interior of a poi-tion of the brachial valve, retaining at (x) the accessory adductor impressions. 



Chemung gi'oup. High Point, N. T. 

 Fig. 30 Posterior view of a specimen ; showing the great disparity in the convexity of the two valves, the 



lower being the bi-achial valve. 

 Fig. 31. Profile of another individual ; showing the gibbosity of the brachial valve. 



Chemung group. Howard, N. Y. 

 Fig. 32. An internal cast of a large brachial valve in which the muscular impressions are unsymmetiically 

 developed. The cast of the cardinal process shows its subdivision on the posterior face. 

 Chemung group. Hicfh Point, N. Y. 

 Compare figures %a-k, Orthis TuUiensis, Palsontology of New York, Volume IV, plate 7. 



* This name was preoccupied by Orthis Missouriensis, Shumard ; Reports I and II of the Geological Survey of Missouri, 

 1855, part ii, page '205, plate c, flgs. ti a, b. That species, if belong-ing to the Ortbid^, should be placed under Oethis as 

 restricted, or under Dinortiiis, thus leaving Orthis Missouriensis of Swallow under the genus Khipodomella 



t The species of Orthis= Schizophoria, described as propinqua, 0. TuUiensis, O impressa, O Iowensis and 

 0. Macfarlanii, present so many features in common that farther study and comparison should be given them to determine 

 the actual value of tlie characters on which the specific distinction has been based, and whether these differences coincide 

 with their geological relations. 



