254 NORTH AMERICAN INDEX FOSSILS. 



radii which are as broad as the spaces between and present an 

 appearance much like the radii of Pecten. 



Trenton of New York, Pennsylvania, Iowa, Minnesota, Ontario, 

 Manitoba. 



173. D. subquadrata (Hall). (Fig. 303, m-o.) Ordovicic. 

 Larger and more subquadrate than the preceding species, the 



sides and front being nearly straight. Surface marked by sharp 

 radiating striae which increase by bifurcation and intercalation. 



Lorraine of the Ohio Valley, Missouri, Illinois, Wisconsin, Min- 

 nesota, Iowa, Manitoba, Anticosti. 



LI. Hebertella Hall and Clarke. 



Cardinal area about equalling the greatest width of the shell, 

 that of the pedicle valve being much the higher. Pedicle valve 

 depressed convex, always less convex than the brachial which is 

 frequently much inflated. Both valves covered with fine, rounded 

 and closely crowded striae or plications which increase by interca- 

 lation ; these are crossed by concentric growth lines. Dental la- 

 mellae continued as a strong ridge around the obcordate muscular 

 area. Cardinal process simple, elongate. Shell impunctate. Ordo- 

 vicic-Siluric. 



A. Small, less than % inch long 175. H. bellirugosa. 



B. More than ]/ z inchlong *. 



* Broad sinus present on pedicle valve I. 



1. Surface striate a. 



a. Striae irregularly bifurcating. Beak moderately high. 



177. H. Occident 'a/is 

 a. Striae stronger and regularly bifurcating. Beak very high. 



178. H. sinuata. 

 I. Surface plicate 174. H. borealis. 



* No sinus present on pedicle valve 176. H. insculpta. 



174. H. borealis (Billings). (Fig. 304, a-c.) Ordovicic. 

 Both valves convex. Hinge line somewhat less than the width 



of the shell below. Brachial valve with a low, undefined mesial 

 prominence ; pedicle valve with a broad and very shallow mesial 

 sinus. Surface covered with about 40 radiating ribs. Much like 

 Plectorthis plicatella, but differs in the presence of the fold and 

 sinus. 



Chazy-Trenton of Kentucky, Tennessee, Wisconsin, Minnesota, 

 Ontario, Quebec 



