PLATE V. 



(Fiirurcs 1-S, 13. 18-2«, iV-JH by E. Emmons; h IJ, H-17, 27, -.'S, ,17-41 by U. 1'. Whitkiki.d.) 



Legend. A- Deltidium. a'. I'ostcriur adihictui- impressions, 



t. Teeth. r. Anterioi' diductor impressions, 



j. Cardinal process. r'. Posterior diductor impressions, 



fp. Crural plate. v. Vascular markings, 



a. Anterior .'nlductor imjjressions. s'. Deltidial plate or eallosity. 



ORTHIS Dalm.\n. 1828. 



l':ige 1S6. 



Orthls, as lestrictcd. 



P.ige 19i. 



"OiJTiris ORTHAMBOMTES (Piinder) " Billing.s.* 



Fig's. 1, '2. Exterior and intei-ioi' views of a pedicle-vidve. X '■^■ 

 Fig. 3. Interior of a liraidiial valve. 



From Ml-. BiLLiNti.s' original specimens. "Point Leuis ; in the uppur part of Limestone No. 2, 

 Quebec group." 



Oktiiis Euryone, Billinsis. 



Fig. 4. Extei-ior of a pedicle-valve; from tlie original specimen. " Point Levis ; in the upper part of 

 Limestone No. 2, Quebec group" 



Ortihs David.soni, de Venieiiil. 



Fig. 5 Interior of a pedicle-\alve ; sliinving the character of the cardinal and muscular areas. 



Fig. 6. Profile of an imlividnal having tlie \ alves in conjunction. 



Fig. 7. A portion of the exterior of the pedicle-valve; showing the fine transverse strije and the oiieiiings 



of sparsely scattered, oblicjue tubules, which penetrate the outer layers of the shell. X 4. 

 Fig. 8. A portion of the cardinal area of the brachial valve ; showing the linear cardinal process, crural 

 plates, thickened deltidi.'il area and the low median ridge. X 3. 

 Upper Silurian limestone. I.sland tf Gfotland. 

 For further iUu.stration, see Davidson, Silurian Brachio, oda, plate xxxv, tigs. IS, 19. 



Ortiiis tricenaria, Conrad. 



Figs. 9-11. Views of the brachial and pedicle-valves and the protile of a specimen with valves conjoined ; 



showing the general exterior characters of this type of .structure. 

 Fig. 12. The interior of a pedicle-valve; showing a slight apical accretion or callosity in the delthyi-ium, 

 and the muscular ai'ea in wiiich the impression of the central adductors is unusually developed. 

 Trenton horizon. Mineral Point TViscousin. 

 Fig. 13. The cardinal area of a pedicle-valv(^ ; showing the character of the apical callosity partially tilling 

 the delthyrium. X 3. 



Trenton horizon. iSt. Paul, Minnesota. 

 Fig. 14. The interior of a brachial valve ; showing the character of the process, crural jilates and muscu- 

 lar area. 

 For further illnstrafion, see Palieontology of New York, Volume I, pl;ite xxxii, fig. 6. 



Orthis costalis, Hall. 



Fig. 15. An internal cast of the pedicle-valve; showing the characters of the muscular area and vascular 



markings. X 2. 

 Figs. 16, 17. Outline protile and full view of another pedicle-valve. 

 Chazy limestone. Chazy, N. Y. 

 For further illustration, see Paleontology of New York, Volume I, plate iv bis, tig. 4. 



PLECTORTHIS, nom. nov. 



Piige 194. 



Orthis plicatella, H;dl. 



Fig. 1,S. A view of the brachial valve of a normal adult, a strongly biconvex shell with a moilerately low 

 cardinal area and strong, distant, simple ribs. 



* ^®® "."'S "" "^i* species on page 221. Tlie American shell differs fi'om the Russian 0. calligramma, var. orthambonites, 

 de verneuil, in its smaller size, fewer an<l coar.ser ribs with line radial striation.s,, iind being thus n quite disiiiict form, 

 it may receive the designation Orthis Panderiana, nom. nov. 



