252 NORTH AMERICAN INDEX FOSSILS. 



Middle and Upper Cambric of Indian Territory, Oklahoma, 

 Texas. 



1 66. P. remnicha Winchell. Cambric. 

 Of medium size, usually slightly transverse. Hinge line varying 



in length from nearly the greatest width of the shell to two thirds 

 the greatest width. Cardinal areas narrow. Brachial valve slightly 

 less convex than the pedicle. Surface marked by bifurcating 

 radiating ribs that vary on shells pi similar size from sixteen in the 

 space of i- inch to three in the same space. 



Saint Croix of Minnesota and Wisconsin, and equivalent forma- 

 tions of Wyoming, Montana, Indian Territory. 



167. P. plicatella Hall. (Fig. 301, d-f.) Ordovicic. 

 Broadly semioval. Valves equally convex without depression 



or elevation. Length and breadth as 3 to 4. Cardinal area nar- 

 row. Plications about 20-28. 



Trenton-Lorraine of New York, Kentucky, Ohio, Wisconsin, 

 Minnesota. 



168. P. whitfieldi(N. H. Winchell). (Fig. 301, g-i.) Ordovicic. 

 Larger than P. plicatella with an almost square outline. Cardi- 

 nal area strongly elevated. Plications numerous. 



Lorraine of Illinois, Iowa, Wisconsin, Minnesota. 



169. P. fissicosta Hall. (Fig. 300, c-d.) Ordovicic. 

 Of medium size. Slightly and nearly evenly convex. Distin- 

 guished especially by the character of the costae which are angular 

 and become two or three times divided toward the margin of the 

 shell. 



Lorraine of Ohio. 



L. Dinorthis Hall and Clarke. 



Differs from OrtJiis (restricted) in the convex brachial valve, in 

 the pedicle valve which is elevated at the umbo and becomes 

 gradually depressed and finally flat or concave, in the dental 

 lamellae being extended around a subquadrate muscular area, and 

 in having an erect cardinal process. Ordovicic. 



170. D. deflecta (Conrad). (Fig. 303, a-d.) Ordovicic. 

 Brachial valve moderately convex. Hinge line usually forming 



the greatest diameter of the shell. Cardinal areas at right angles 

 to each other with broadly triangular delthyrium partially covered 





