BRACHI0P0DA—0RTHID2E. 



251 



FlG. 301. a-c, Orthis tricenaria ; d-f, Plectorthis plicatella ; g-i y P. whitfieldu 

 (Minn. Geol. Surv. ) 



164. 0. flabellites Foerste. (Fig. 302.) Siluric. 

 Semioval, with long hinge line. The coarse plications crossed 



by concentric growth lines. 



Clinton and Niagara of New York, Kentucky, Ohio, Indiana, 

 Wisconsin, Ontario. 



XLIX. Plectorthis Hall and Clarke. 

 Diners from Orthis (restricted) in the subequal and convex 

 valves, in having less simple plications and in the comparatively 

 low cardinal area of the pedi- 

 cle valve. Ordovicic - De- 

 vonic. 



165. P. indianola Walcott. 



Cambric. 

 Small, transverse. Hinge 

 line varying from slightly less 

 to one fifth greater than the 

 width of the shell below. 

 Pedicle valve about twice as 

 convex as the brachial, with 

 or without a mesial sinus. Brachial valve with a shallow or deep 

 mesial sinus. Cardinal areas low. Surface marked with strong or 

 fine radiating ribs or striae. 



Fig. 302. Orthis flabellites. 



