PLATE LIV. 



(Figures 1-3, 23-86, 30, S5-37 by K. Emmons; 6, 9-11, 13-16, 21, 22 27, 31-34 by G. 1!. SjmI'Son; 4, 5, 7, 8, 12, 17-2<), 28, 

 29, 38-40, 47, 48 by B. P. Whitfield; 41-46 copies.) 



Genus ZYGOSPIRA, Hall. 



Page 154. 



Zygospiba reourvirostka, Hall. 



Fig¥. 1-3. Dorsal, ventral and cai-dinal views of the exterior of an average example. X 3. 



Trenton limestone. MiddlemUe, N. T. 

 Figs. 4, 5. Dorsal and profile views of a more finely plicated specimen. X 3. 



Trenton limestone. SavannaJi, Illinois, 

 Y\g. 6. Dorsal view of a shell much more finely plicated than the foregoing, and with a bi-oader median 

 sinus on the brachial valve. This is the prevailing form at this locality. 



Trenton horizoji. Frankfort, Kentucky. 



Zygospira modesta (Say), Hall. 



Fig. 7. Doreal view of the exterior ; showing the character of the plication. X 3. 

 Pig. 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 group. Versailles, Indiana. 

 Kg. 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 appearance and sharp definition of a true area 

 ae here represented ; the upper margin nevei- showing such angularity. 

 Hudson River group. Near Cincinnati, Ohio. 



Zygospira Kentuckiensis, James. 



Figs. 11, 15, 16. Ventral, doreal and profile views of an average specimen, natural size ; showing the exterior 

 charactere and large size of this form. 



Hudson River group. Oldham county, Kentucky. 



Zygospira Cincinnatiensis, Meek. 



Figs. 13, 14. Venti-al and doreal views of the exterior ; showing the character of the plication and the un- 

 equal size and distribution of the ribs on fold and sinus. X 2. 

 Hudson River group. Cincinnati, Ohio. 



Zygospira (Orthonom^ea) erratica, Hall. 



Fig. 17. The interior of a bi-acbial valve ; showing the bilobed cardinal process, muscular and vascular 

 impressions. 



Fig. 18. The interior of a pedicle-valve ; showing the open delthyrium, deep, posteriorly situated muscular 

 scar, and the lateral impressions at the sides of this scar The impressions in the pallial region 

 which ai-e represented aa branching sinuses are believed to be traces of the primary lamellse 

 of the spiral coils. Figs. 17 and 18 are drawn from gutta-percha impressions taken from nat- 

 ural castfi of the interior. 



Fig. 19. Ventral view of a natural cast of the interior ; showing the muscular scare 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. I>rift blocks in central and western New York. 



Fig. 21. The exterior of a pedicle-valve ; showing its contour and the tine surface plication. 

 Hudson River group. Hamilton, Ontario. 



Fig. 22. The interior of a pedicle-valve ; showing the cardinal process and muscular scars. From a gutta- 

 percha mould. 



Fig. 23. An enlargement of the cai-dinal process of the same specimen ; showing its bilobate character. X3. 

 Hudson River group, Ihift blocks of central New Yoi-k. 



