Fig. 10. An enlargement of the cardinal area to show the denticulated car- 
dinal edge of the pedicle-valve. The surface of the shell on the 
area is somewhat exfoliated, exposing the series of vertical canals, 
each of which terminates in a denticle. The margin of the bra- 
chial valve shows a series of small sockets corresponding to the 
denticles. 
Fig. 11. Dorsal view of a large specimen with rather short hinge. 
Chester limestone. Chester, Illinois. 
SPIRIFER opimus, Hall. 
Figs. 12, 18. Dorsal and profile views of a normal individual. 
Coal measures. Jowa. 
Spirirer Keroxour, Hall. 
Fig. 14. Ventral view of a normal example. 
Fig. 15. Dorsal view of a similar specimen. 
Keokuk group. Keokuk, Iowa. 
Sprrirer Leipyi, Norwood and Pratten. 
Figs. 16, 17. Profile and dorsal views of a normal individual. 
Chester limestone. Chester, Illinois. 
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