4 oo NORTH AMERICAN INDEX FOSSILS. 



Differs from P. constricta in its smaller size and more elongate 

 form. 



Common in Hamilton of New York, Pennsylvania and Ontario. 

 Marcellus : New York. Ithaca : New York, Pennsylvania. 



88. P. emarginata (Conrad). (Fig. 510,^-/.) Devonic. 

 Posterior margin deeply emarginate owing to the strongly 



marked ridge and depression on the umbonal slope. Concentric 

 striae strengthened into distant lamellose ridges with finer striae 

 between. 



Hamilton : New York and Appalachian region. Portage : 

 New York. 



89. P. constricta (Conrad). (Fig. 510, a t b.) Devonic. 

 Basal margin rounded anteriorly and slightly constricted pos- 

 teriorly. Surface striae thread-like, often obsolescent. 



Hamilton : Appalachian and interior regions. Portage and 

 Chemung of New York and Pennsylvania. 



90. P. brevis Hall. (Fig. 510, d.) Devonic. 

 Similar to P. constricta in general outline but is more gibbous, 



less curved along the basal margin, and only very slightly con- 

 stricted posteriorly. 



Chemung : New York, Pennsylvania. 



91. P. marshallensis Winchell. {P. truncata Hall.) (Fig. 510, e.) 



Mississippic. 



Distinguished by the nearly parallel basal and cardinal margins, 

 the short anterior portion, the truncate posterior margin, and the 

 fine striae, which though in parts crowded do not rise into varices. 



Waverly of Ohio : Marshall group of Michigan. 



92. P. sulcatina (Conrad). (Fig. 510,7.) 



Upper Devonic and Mississippic. 



Valves gibbous, regularly rounded, with no marked truncation 

 or constriction. Concentric elevations sharp and lamellose inter- 

 spersed with finer concentric striae. 



Chemung : Iowa. Waverly : Ohio, Michigan. 



XXIX. Leda Schumacher. (Nucula?ia Link.) 

 Porcelainous, often somewhat crescent-shaped and produced 

 posteriorly. Rounded anteriorly. Beaks often tumid, turned pos- 

 teriorly. Surface concentrically striated. Hinge with a line of 



