PLATE 38. 
. 1-25. Spirifera medialxs.... 
1. Dorsal view of a small and very symmetrical specimen. 
2, 3. Dorsal and cardinal views of a larger specimen, having the extremities of the hinge line 
extended; fig. 3, shows the cardinal area and foramen. 
4-6. Dorsal, cardinal and profile views of a more ventricose specimen, with a narrow incurved 
area and strong concentric lines of growth. 
7, 8. Dorsal and cardinal views of a specimen of medium size, having a higher area, which is 
but slightly incurved. 
9-11. Dorsal, profile and ventral views of a specimen having a high area and a semicircular 
dorsal valve. 
12, 13. Dorsal and cardinal views of a specimen of slightly Unsymmetrical form and rather 
strong ribs. 
14-16. Dorsal, front and ventral views of a large specimen., having a small area and 
slightly projecting ventral beak. 
17 - 20. Dorsal, ventral, front and profile views of another specimen corresponding in size with 
the last, but having a higher area, more projecting ventral beak, and higher mesial fold, 
presenting a strong contrast between figs. 17 and 14, and between 19 and 15. 
21, 22. Cardinal views of two individuals with high areas, showing considerable variation 
in the proportions of the deltidium; contrasting strongly in these respects with fig. 13. 
23, 24. Interiors of dorsal valves, showing the muscular impressions, socket plates, etc. 
25. Interior of a ventral valve, showing the muscular impression, cardinal area, teeth, and the 
eallosity filling the apical portion of the deltidial opening. 
