360 



NORTH AMERICAN INDEX FOSSILS. 



Mcristella. Hinge plate rises vertically from the bottom of the 

 pedicle valve ; its anterior face bears a low, median plication on 

 each side of which posteriorly rise the two short crura. The pos- 

 terior face of the hinge plate bears a deep concavity. Devonic. 



427. P. unisulcata (Conrad). (Fig. 475-) Devonic. 



Subtrigonal, wider in front. Median sinus of pedicle valve oc- 

 cupying nearly the whole width of the valve and bounded on each 

 side by an angular fold; umbo prominent and incurved over that 

 of brachial valve. Brachial valve gibbous in middle, the promi- 

 nent mesial fold marked by a sinus extending to the beak. Sur- 

 face bearing a few concentric growth lines and rarely concentric 

 folds. 



Oriskany-Hamilton of New York, Ohio, Kentucky and Ontario. 



Hg. 475. Ptnta*onia unisulcata (Pal. N. Y., IV.) 



