PLATE LIV. 
(Figures 1-3, 23-26, 30, 35-37 by E. Emmons; 6, 9-11, 13-16, 21, 22 27, 31-34 by G. B. Simpson; 4, 5, 7, 8, 12, 17-20, 28, 
29, 38-40, 47, 48 by R. P. WHITFIELD; 41-46 copies.) 
Genus ZYGOSPIRA, Hall. 
Page 154. 
Zygospira recurvirostka, Hall. 
Figs. 1-3. Dorsal, ventral and cardinal views of the exterior of an average example. X 3. 
Trenton limestone. Middlemlle, N. Y. 
Figs. 4, 5. Dorsal and profile views of a more finely plicated specimen. X 3. 
Trenton limestone. SavannaTt, Illinois. 
Fig. 6. Doi'sal view of a shell much more finely plicated than the foregoing, and with a broader median 
sinus on the brachial valve. This is the prevailing form at this locality. 
Trenton horizon. Frankfort, Kentucky. 
Zygospira modesta (Suy), Hall. 
Fig. 7. Dorsal view of the exterior ; showing the character of the plication. X 3. 
Fig. 8. A profile of a somewhat larger individual. X 3. 
Hudson River group. Near Cincinnati, Ohio. 
Fig. 9. The interior of a brachial valve ; showing the bilobed cardinal process. X 2. 
Fig. 10. A portion of the same specimen enlarged to show more distinctly the structure of this process. X 6. 
Hudson River grouxi. Versailles, Indiana. 
Fig. 12. An enlargement of the cardinal region ; showing the deltidial plates and the encroachment of the 
foramen upon the umbonal portion of the shell. In this specimen the false cardinal area is 
unusually developed, but it does not take on the ajipearance and shaip definition of a true area 
as here represented ; the ujiper margin never showing such angvdarity. 
Hudson River group. Near Cincinnati, Ohio. 
Zygospira Kentuckiensis, James. 
Figs. 11, 15, 16. Ventral, dorsal and profile views of an average s^iecimen, natural size ; showing the exterior 
characters and large size of this form. 
Hudson River group. Oldham county, Kentucky. 
Zygospira Cincinnatiensis, Meek. 
Figs. 13, 14. Ventral and dorsal views of the exterior ; showing the character of the iilication and the un¬ 
equal size and distribution of the ribs on fold and sinus. X 2. 
Hudson River group. Cincinnati, Ohio. 
Zygospira (Orthonom^.a) erratica, Hall. 
Fig. 17. The interior of a brachial valve ; showing the bilobed cardinal iirocess, muscular and vascular 
imjiressions. 
Fig. 18. The interior of a xiedicle-valve ; showing the oj^en delthyrium, deeii, posteriorly situated muscular 
scar, and the lateral impi-essions at the sides of this scar The imin-essions in the piallial region 
which are represented as branching sinuses are believed to be traces of the .primary lamellae 
of the spiral coils. Figs. 17 and 18 are drawn from gutta-percha imiiressions taken fi’om nat¬ 
ural casts of the interior. 
Fig. 19. Ventral view of a natural cast of the interior; showing the muscular scars of the pedicle-valve. 
X 2. 
Fig. 20. A natural cast of the brachial valve ; showing the arrangement of the muscular scars. X 2. 
Hudson River group. Drift blocks in central and western New York. 
Fig. 21. The exterior of a pedicle-valve ; showing its contour and the fine surface jilication. 
Hudson River grouj). Hamilton, Ontario. 
Fig-. 22. The interior of a pedicle-valve; showing the cardinal xu-ocess and muscular sears. From a gutta¬ 
percha mould. 
Fig. 23. An enlargement of the cardinal process of the same sxiecimen ; showing its bilobate character. X 3. 
Hudson River group. Drift blocks of central New York. 
