PLATE V. 



(Figures 1-S, 13, 18-2B, 2H-36 by E. EJIMONS; il-li, 14-17, 27, 28, 37-41 by It. P. Whitfield.) 



Legend. ^. Deltidii.im. 

 t. Teeth. 



j. Cardinal process, 

 fp. Crural plate, 

 a. Anterior adductor impressions. 



a'. Posterior adductoi' impressions. 



r. Antei-ior diductor impressions, 

 r'. Posterior diductor impressions. 



V. Vascular markings. 



s'. Deltidial plate or callosity. 



Figs. 

 Fig. 



Fig. 



ORTHIS Dalman. 1828. 



Page 186. 



Orthis, as restricted. 



Page 192. 



" Orthis orthambonites (Pander) " Billings.* 



1, 2. Exterior and inteiior views of a pedicle-valve. X 3- 



3. Intei-ior of a brachial valve. 



From Mr. Billings' original specimens. "Point Levis; in the upper part of Limestone No. 2, 

 Quebec group." 



Orthis Eurtone, Billings. 



4. Exterior of a pedicle-valve; from the original specimen. " Point Levis ; in the upper part of 



Limestone No. 2, Qnebec group." 



Orthis Davidson i, de Verueuil. 



Fig. 5. Interior of a pedicle-valve ; showing the character of the cardinal and muscular areas. 



Fig. 6. Profile of an individual having the valves in conjunction. 



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



of sparsely scattered, oblique 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 pi-ocess, crural 

 plates, thickened deltidial area and the low median ridge. X 3. 

 Upper Silui-ian limestone. Liland of Gotland. 

 For further illustration, see Davidson, Silurian Bi'achioj^.oda, plate xxxv, figs. IS, 19. 



Orthis tricbnaeia, Conrad. 



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



showing the genei-al exterior characters of this type of structure. 

 Fig. 12. The inteiior of a pedicle-valve; showing a slight apical accretion or callosity in the delthyrium, 

 and the in uscular area in wliich the impression of the central adductors is unusually developed. 

 Trenton horizon. Mineral Point. Wisconsin. 

 Fig. 13. The cardinal area of a pedicle-valve ; showing the character of the apical callosity partially filling 

 the delthyrium. X 3. 



Trenton horizon. St. Paul, Minnesota. 

 Fig. 14. The intei-ior of a brachial valve ; showing the character of the process, crural plates and muscu- 

 lar ai-ea. 

 For further illustration, see Palaeontology of New York, Volume I, plate xxxli, fig. 6. 



Orthis costalis, Hall. 



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



markings. X 2. 

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

 Chazy limestone. CJiazy, N. Y. 

 For further illustration, see Paljeontology of New York, Volume I, plate iv bis, fig. 4. 



PLECTORTHIS, nom. nov. 



Fage 194. 



Orthis plicatella, Hull. 



Fig. IS. A view of the brachial valve of ii noi-mal adult, a strongly biconvex shell with a moderately low 

 cardinal ai'ea and strong, distant, simple ribs. 



* See note on this species on page 221. Tlie American shell differs from the llussian 0. caltifframma, var. oriliambonites, 

 de Verneuil, in its smaller size, fewer and coarser ribs with line radial striations,. and being thus a quite distinct form, 

 it may receive the designation Orthis Paiideriana, nom. uov. 



