258 NORTH AMERICAN INDEX FOSSILS. 



A. Height and breadth about equal. Shell very gibbous 181. P. crassa 



B. Breadth exceeding height * 



* Hinge line rounded at extremities and less than the greatest width of shell. 



183. P. Ivnx. 



* Hinge line extended I. 



I. Two plications in sinus 185. P. biforata. 



I. Three or more plications in sinus a. 



a. Hinge line usually with mucronate extremities. Plications many. i / . 



l / . Fold pronounced 185. P. biforata. 



l / . Fold not pronounced 182. P. aculilirata. 



a. Hinge line extremities not mucronate. Plications wide and few. 



184. P. laticosta. 



181. P. crassa: James. (Fig. 308, a-b.) Ordovicic. 

 Hinge line short with rounded extremities. Shell as wide as 



long. Both valves very gibbous. [Cf. P. costata Pander.) 

 Lorraine of Ohio, Minnesota, Manitoba. 



182. P. acutilirata (Conrad). (Fig. 308, e-i.) Ordovicic. 

 Usually much extended on hinge line which is often mucronate. 



Plications simple, from 10 to 18 on each side of fold and nnus 

 with usually only three in the sinus and four on the fold. 

 Lorraine of Indiana, Ohio, Missouri and Louisiana. 



183. P. lynx (Eichwald). (Figs. 308, c-d ; 309.) Ordovicic. 

 Large, wider than long. Hinge line usually less than the 



greatest width of the valves, with obtusely angular extremities. 

 Plications (16-24) angular, with three (1-7) in the mesial sinus. 

 Trenton of New York, Lorraine of Ohio, Kentucky, Indiana, etc. 



184. P. laticosta Meek. Ordovicic. 

 Of medium size, wider than long, with hinge line forming 



greatest width of shell. Plications large and few, 5 to 7 on the 

 lateral slopes and 1 to 3 on the sinus and 2 to 4 on the fold ; those 

 marking fold and sinus are irregular and of different sizes. 

 Lorraine and Richmond of Ohio valley. 



185. P. biforata (Schlotheim). Ordovicic and Siluric. 

 Semielliptical or subquadrate, wider than long. Cardinal ex- 

 tremities varying from submucronate to obtuse. Usually about 2 

 (2-9) of the sharp plications are in the sinus while about 3 (3-10) 

 mark the fold, rapidly increasing in strength as they approach the 

 front. 



Trenton of Hudson and Champlain valleys and Clinton of New 

 York, Ohio, Kentucky and Indiana. 





