PLATE V. 
(Figures 1-8,13, 18-26, 29-36 by E. Emmons; 9-12, 14-17, 27,2 
Legend. A- Deltidium. 
t. Teeth. 
j. Cardinal process, 
fp. Crural plate. 
a. Anterior adductor impressions. 
, 37-41 by 1{. P. WHITFIELD.) 
a'. Posterior adductor impressions, 
r. Anterior diductor impressions, 
r'. Posterior diductor impressions, 
v. Vascular marking’s, 
s'. Deltidial plate or callosity. 
ORTHIS Dalman. 1828. 
Page 186. 
Orthis, as restricted. 
Page 192. 
“Orthis orthambonites (Pander) ” Billings.* 
Figs. 1, 2. Exterior anil interior views of a pedicle-valve. X 3. 
Fig. 3. Interior of a brachial valve. 
From Mr. Billings’ original specimens. “ Point Levis; in the upper part of Limestone No. 2, 
Quebec group.” 
Orthis Euryone, Billings. 
Fig. 4. Exterior of a pedicle-valve; from the original specimen. “ Point Levis; in the upper part of 
Limestone No. 2, Quebec group.” 
Orthis Davidson i, de Verneuil. 
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 striae 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 process, crural 
plates, thickened deltidial area and the low median ridge. X 3.. 
Upper Silurian limestone. Island of Gotland. 
For further illustration, see Davidson, Silurian Brachio* 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 delthyrium, 
and the muscular area in winch 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 interior of a brachial valve ; showing the character of the process, crural plates and muscu¬ 
lar area. 
For further illustration, see Palaeontology of New York, Volume I, plate 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 profile and full view of another pedicle-valve. 
Chazy limestone. Chazy, N. T. 
For further illustration, see Palaeontology of New York, Volume I, plate iv bis, fig. 4. 
PLECTORTHIS, nom. nov. 
Page 194. 
Orthis plicatella, Hall. 
Fig. 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 page 221. The American shell differs from the Russian 0. calligramma, var. orthambonites, 
de Verneuil, in its smaller size, fewer and coarser ribs with line radial str.iations,. and being thus a quite distinct form, 
it may receive the designation Orthis Panderiana, nom. nov. 
