PLATE LIV. 



(Figuies 1-3, 23-26, 30, 35-37 by E. Emmons; K, y-11, 13-16, 21, 22 27, :il-34 liy G. 1!. simI'SOn; 4, 5, 7, S, 12, 17-20, 28, 

 29, 38-40, 47, 48 by R. P. Whitfield; 41-t« copies.) 



Genus ZYGOSPIRA. Hall. 



Pago 154. 



Zygospika recurvikostua, Hall. 



FigH. 1-3. Dorsal, ventral and cardinal views of the exterior of an average examiile. X ■i- 



Trenton limestone. Middleville, N. Y. 

 Figs. 4, 5. Dorsal and jirofile views of a more tinely plicated sjiecitnen. X 3. 



Trenton limestone. Savannah, Illinois. 

 Fig. H. Dorsal view of a shell much nioi-e finely plicated than the foi-egoing, and with a bi'oader median 

 sinus on the brachial valve. This is the pi'evailing form at this loi'ality. 



Trenton horizon. Frankfort, Kentucky. 



Zygospika modesta (Say), 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 pi-ocess. 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. Versailhs, Indiana. 

 Fig. 12. An enlargement of the cardinal region ; showing the dellidial 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 sharji definition of a ti'ue area 

 as here represented ; the upper margin nevei' showing such angularity. 

 Hudson River group. Near Cincinnati, Ohio. 



Zygospira Kentuckiensi.s, Jainc!?. 



Figs. 11, 15, 16. Ventral, dorsal and profile views of an average si)ecimen, 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 charactei- of the jilication ami the un- 

 equal size and distrilmtion of the ribs on foM and sinus. X 2. 

 Hudson River group. Cincinnati, Ohio. 



Zygospira (Orthonom^a) ekratica. Hall. 



Fig. 17. The interior of a brachial valve ; showing the bilolied cardinal process, muscular and vascular 

 impressions. 



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

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

 which are i-epresented as branching sinuses are believed to be ti'aces of the primary lamellae 

 of the spii'al coils. Figs. 17 and 18 are drawn from gutta-percha impressions taken from 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. .\ natural cast of the brachial valve ; showing the ai-rangement of the muscular scars. X 2. 

 Hudson River group. Drift blocks in ceiitral and westei-n 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 cardinal process of tlie same specimen ; showing its bilobate character. X 3. 

 Hudson River group. Drift blocks of central Nnu York. 



