PLATE V. 



(Fifnires 1-S, 13. IS-2U, 2U-36 by K. Em mom 



Legend. A. Deltidium. 

 t. Teeth. 



j. Cardinal process. 

 fp. Crural plate, 

 a. Anterior adductor impi'essions. 



;x, .i7-4l by It. I'. \VlHTFlIil,D.) 



a'. Posterior adductoj' impressions. 



r. Anfei'ioi' didiictor impressions. 

 !•'. Po.slerior diihu'tor impressions. 



V. Viiscular markings, 

 s'. Deltidial plate or callosity. 



ORTHIS Dalman. 1828. 



Page 1S6. 



Orthis, iis restricted. 



P.tge 192. 



"Orthis okthambomtes (Pander) " Billings.* 



Figs. 1, 2. Exterior and iiitei'ioi- views of a iieiiicle-valve. X 'A- 

 Fig-. 3. Interior of a bi'achial valve. 



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

 Quebec group." 



Orthis Euryone, Billings. 



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

 Limestone No. 2, Quebec group" 



Orthi.s Davidsoni, de Verneuil. 



Fig. 5 Intei-ior of a pedicle-valve ; t^howing the charactei- of the cardin.il and muscular areas. 



Fig. 6. Profile of an individual having tlie 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, oVjlicpie tnbu es, which peneti-ate the outer layers of the shell. X 4. 

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

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

 Ulipei- Silurian limestone. Island of Crotland. 

 For further illustration, see Davidson, Silurian Brachioj. oda, plate xxxv, figs. 18, 19. 



Orthis tricenaria, Conrad. 



Figs. 9-11. Views of the brachial and pedicle-valves and the profile 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 ni uscular area in which the impression of the central adductoi-s is unusually developed. 

 T:-enton 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 hoi-izon. St. Paul, Ninnesota. 

 Fig. 14. The interior of a In-achial valve ; showing the character of the jirocess, crural plates and muscu- 

 lar area. 

 For further illustration, see PaliBontology of New Yoi-k, Volume I, plate xxxii, fig. 6. 



Orthis costalis, Hall. 



Fig. 15. An internal cast of the peilicle-valve ; showing the characters of the mn.scular area and vascular 



maikings. X 2. 

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

 Chazy limestone. Chazy, N. Y. 

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



Fig. 



PLECTORTHIS, nom. nov. 



Puge 194. 



Orthis plicatella, Hall. 



IS. A view of the brachial valve of a normal adult, a strongly biconvex shell with a moderately low- 

 cardinal area and strong, distant, simple ribs. 



• See note on this species on pivge 221. The American shell differs from the Russian O. calligramma, var. orthambonites, 

 tie Verneuil, in its smaller size, fewer ami coai-ser ribs with due radial striations,. ami being thus a quite distinct form, 

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



