244 



NORTH AMERICAN INDEX FOSSILS. 



rounded. No mesial depression present. Hinge line short, form- 

 ing scarcely more than half the width of the shell. Surface marked 

 with very fine, thread-like striae. Spine bases scarcely visible. 

 Kinderhook of Iowa, Missouri, Utah. 



148. P. burlingtonensis Hall. (Fig. 295, a-c.) 



Lower Carbonic. 



Pedicle valve extremely ventricose with beak incurved to near 



the middle of the valve; median sinus present, extending from 



near the beak to the front. Brachial valve moderately concave 



above and abruptly geniculate in the middle. Surface marked by 



Fig. 295. a- 

 Whitfield. ) 



■c, Productus burlingtonensis ; d-g, P. biseriatus. (After Hall and 



radiating, bifurcating ribs crossed by fine concentric striae. Strong 

 wrinkles present on the upper portion of the shell and scattered 

 spines on the middle and lower parts. 



Burlington of Illinois, Iowa, Missouri, Utah. 



149. P. biseriatus Hall. (Fig. 295, d-g.) Lower Carbonic. 



Small, longitudinally ovate ; hinge line scarcely as long as the 

 width of the shell below. Pedicle valve extremely gibbous, with- 

 out sinus and marked by five or six elevated, distant, concentric 

 undulations which bear on their upper margins a row of elongate 

 nodes and below this many smaller granulations. Beak attenuate 

 and extremely arcuate. Brachial valve flat near the front and 

 marked by 8 or 9 concentric bands set with granulations. 



St. Louis of Kentucky, Indiana, Illinois, Missouri. 



