432 NORTH AMERICAN INDEX FOSSILS. 



1 88. B. praecursa Ulrich. (Fig. 565.) Ordovicic. 

 Radii 38-42. Like B. radiata but differs in its less obliquity, 



longer hinge line, and especially in the flattening of its anterior 

 side. 



Lorraine of New York, Ohio, Kentucky. 



189. B. radiata (Hall). (Figs. 562, c, 566.) Ordovicic. 

 Surface marked by strong radiating ribs which are often grooved 



on the upper portion of the shell, giving them a duplicate char- 

 acter. 



Cincinnati Group : New York-Iowa and Wisconsin. 



FlG. 565. Byssonychia prtccursa. (Pal. FlG. 566. Byssonychia radiata. (Pal. 

 Ohio, VII.) Ohio, II.) 



LVII. Allonychia Ulrich. 

 Like Byssonychia in the possession of a byssal opening but with 

 neither cardinal nor lateral teeth. Likewise more erect than other 

 Ambonycliia-Yikz forms. Ordovicic. 



190. A. jamesi (Meek). Ordovicic. 

 Not winged posteriorly but with a lobe-like projection anteriorly. 



Surface marked with regular radiating ribs. Type of genus. 

 Cincinnati Group of Ohio. 

 * 



LVIII. Mytilarca Hall. 

 Differs from PLethomytilus in the presence of cardinal teeth. 

 Devonic-Miss. 



191. M. chemungensis Conrad. (Fig. 567, a, b.) 



Devonic. 

 Shell convex, of medium size. Beaks elevated. 

 Chemung of New York. 



