BRACHIOPODA—PROTREMATA. 223 



cular area subcircular and deeply excavated with an elevated ridge 

 nearly surrounding it. Brachial valve slightly concave at umbo, 

 rapidly becoming convex. Ordovicic-Carbonic. 



A. Shell nearly flat 95. S. billingsi. 



B. Shell strongly concavo-convex, i. e., pedicle valve concave and brachial convex.. *. 



* Shell triangular in outline. Cardinal extremities marked by oblique wrinkles. 



98. S. trilobata. 



* Shell transversely extended with acute hinge extremities (i). 



(1) Large, over 1*4. inch wide • 96. S. incurvata. 



(1) Small, less than \y 2 inch wide 97. S. trentonensis. 



* Shell subquadrate with more or less rectangular hinge extremities (2) 



(2) Of medium size, about 1J/3 inches wide 99. S. neglecta. 



(2) Small, less than 1^3 inches wide a. 



a. Shell not wrinkled on cardinal margin 100. S. rugosa. 



a. Shell obliquely wrinkled on cardinal margin. 



\OOa. S. rugosa subtenta. 



95. S. billingsi Winchell and Schuchert. Ordovicic. 

 Small, usually not exceeding one inch in width. Both valves 



nearly flat ; beaks scarcely distinct from the cardinal area. Bra- 

 chial valve marked by a very faint mesial depression for about half 

 the way from the beak to the front of the shell. Radiating striae 

 fine and usually of different sizes. 



Trenton of Minnesota, Ontario, Manitoba. 



96. S. incurvata (Shepard). (Fig. 271, a-d.) Ordovicic. 

 Of medium size, usually about one and three fourths inches 



wide, with greatest width along the hinge-line and with cardinal 

 extremities acutely angular and deflected. Pedicle valve strongly 

 concave ; brachial quite strongly convex. Radiating striae fine, 

 crowded, alternating in size and crossed by numerous concentric 

 lines and a few stronger growth lines. 



Trenton of New York, Kentucky, Tennessee, Missouri, Iowa, 

 Wisconsin, Minnesota, Manitoba. 



97. S. trentonensis Winchell and Schuchert. (Fig. 272, a.) 



Ordovicic. 



Valves thin. Radiating striae delicate. Cardinal margins 

 marked by oblique wrinkles. Internal markings undefined. 



Trenton of New York, Kentucky, Tennessee, Wisconsin, 

 Minnesota. 



98. S. trilobata (Owen). (Fig. 271, g-k.) Ordovicic. 

 Hinge line extended. Brachial valve broadly trilobate and very 



gibbous in front. Radiating striae fine and equal. 

 Trenton of Iowa, Minnesota, Manitoba. 



