Plate II 
Plate to show the evolution of Spirifer centronatus Winchell above, from Spirifer albapinensis 
Hall and Whitfield, below. The illustrations are arranged somewhat according to the thickness of 
the strata in which they occur, S. albapinensis occurs in locality 35, 215 feet thick, to the exclusion 
of S. centronatus. The latter species appears in locality 34 and continues in great abundance from 
there up through locality 22, a vertical distance of 775 feet of limestones and shales. The young 
in the higher beds show the broad plications bounding the median sinus characteristic of S. alba- 
pinensis. No specimen collected from localities 34 and 35 was sufficiently well preserved for illus- 
tration. 
Figures 1, 2, 3. Pedicle views of Spirifer centronatus, showing change of plications from youth 
to maturity. Section 2, locality 23. (Page 51.) 
Figures 4, 5, 6. The same. Section 2, locality 25. (Page 51.) 
Figures 7, 8, 9. The same. Section 2, locality 29. The plications bounding the median sinus 
remain broad and strongly developed for a longer time than in the higher beds. (Page 51. ) 
Figures 10, 11. The same. Section 2, locality 32. (Page 51.) 
Figure 12. A rather small but typical example of Spirifer albapinensis. Our larger specimens 
from this locality are too imperfect for illustration. Section 2, locality 35. (Page 49.) 
Figures 13, 14. Young and mature specimens of Spirifer albapinensis. Section 2, locality 35. 
(Page 49.) 
