282 



XORTH AMERICAN INDEX FOSSILS. 



242. R. capax (Conrad). (Fig. 343, h-k.) Ordovicic. 



Beak of pedicle valve closely incurved over that of brachial, 

 Apex of pedicle valve usually perforated by a small pedice open- 

 ing. Three (rarely 4) angular plications occupy the very promi- 





^r>. 



Fig. 343. a-d, RhyncJiotrema ineqaivalve (Min. Survey) ; e-g, Rhynchotrema 

 dentatum (Ind. Survey) ; h-k, Rhynchotrema capax (Ind. Survey) ; /-;//, Rhyncho- 

 tremajxinslii (Min. Survey). 



nent sinus and 4 the corresponding fold on the brachial valve. 

 The angular radiating plications are crossed by many strongly zig- 

 zag growth lines. 



Lorraine ; widely distributed throughout North America. 

 243. R. ainslii N. H. Winchell. (Fig. 343, l-m.) Ordovicic. 



Differs from R. incequivalve in being usually larger, more trans- 

 verse, and in having 28 to 34 plications with 6 to 8 on the fold. 



Trenton of Iowa and Minnesota. 



LXXV. Rhynchotreta Hall. 

 Elongate triangular, marked with angular plications. Beak of 

 pedicle valve acuminate and produced beyond that of opposite 



