PLATE 39. 
1-12. Spirifera l^evis. 
1. View of a cast of a dorsal valve of a small individual. 
2. Cast of a ventral valve .of a small individual, showing the muscular impression. 
3. Dorsal view of an imperfect specimen, showing the ventral area, and the deltidium in place. 
4. Cast of a dorsal valve of a large individual. 
5. Cardinal view of a ventral valve, showing the deltidium in place. 
6. Interior of a ventral valve, showing the area, muscular impression, with the deltidial plate. 
covering the upper part of the foramen. 
7. Exterior of a large ventral valve preserving the shell and showing a remarkably wide suban- 
gular sinus, with obscure plications on the sides of the shell. 
8-11. Casts of ventral valves, showing considerable variation in the size and proportions of 
the muscular impression. 
12. Cardinal view of a cast of a ventral valve. 
