330 



XORTH AMERICAN INDEX FOSSILS. 



353. S. angustus Hall. (Fig. 420, b.) Devonic. 



Valves with great lateral extension and pronounced inequality ; 



the pedicle valve forming nearly the entire thickness of the shell. 



Brachial valve flat with narrow 

 cardinal area and low mesial 

 fold. Plications fine, from 48 to 

 56 on each valve. 



Hamilton and Portage of 

 New York, Wisconsin. 



354. S. mucronatus Conrad. 

 (Fig. 421.) Devonic. 



Hinge line extended and of- 

 ten mucronate, giving the shell 

 a width of from two to four 

 times the length or greater. 

 Cardinal areas low. Fold in 

 brachial valve often flattened or 

 grooved. Radiating plications 

 numerous, crossed by lamellose lines of growth which are often 

 crowded near the front. Often a plication in sinus. 



Marcellus, Hamilton and Chemung of New York, Pennsylvania, 

 Maryland, Virginia, Wisconsin, Ontario. 



Fig. 420. a, Spirifer audaculus 



Spirifer anguslus (Pal. N. Y. , IV.). 





'^itiikm- 



Fig. 421. Spirifer mucronatus, showing some of the varieties (J'al. N. V., IV. ). 



