6o8 



NORTH AMERICAN INDEX FOSSILS. 



Elliptical to subcircular, moderately high, with beak slightly 

 incurved. Concentric folds strong on the convex side, nearly 

 absent on concave. 



Lower Cambric (Georgian) of Troy, N. Y.; Bic Harbor, Que- 

 bec ; l'Anse au Loup, Labrador, and (?) Etcheminian of New- 

 foundland. 



VII. Scenella Billings. 



Conical, often high, shells with subcentral apex and radial pli- 

 cations. Muscular scars forming a circle above the mid-height. 

 Cambric-Devonic. 



20. S. reticulata Billings. (Fig. 811, a, b.) Cambric. 



Fig. 810. Helcionella ru- 

 gosa, showing variations. (After 

 Walcott. ) 



b 



Fig. 811. a, b, Scenella reticulata, en- 

 larged ; c, d, S. retusa. (After Walcott, ioth 

 Ann. Rep. U. S. G. S.) 



Small, with excentric apex, and an obscure carina extending 

 from apex to one side of margin. 



Etcheminian of Newfoundland and Massachusetts. 



21. S. retusa Ford. (Fig. 8n,r, d.) Cambric. 

 More nearly circular, smaller, beak subcentral, front slope con- 

 cave ; faint concentric and longitudinal striae present. 



Georgian of Troy, N. Y., Bic Harbor, Canada. 



22. S. SUperba Billings. Ordovicic. 

 Large, ovate to circular, diameter up to 90 mm., height some- 

 what over half diameter, apex subcentral ; radiating plications fine, 

 sharp. 



Stones River of Minnesota ; Black River of eastern Canada. 



23. S. montrealensis (Bill.). (Figs. 812, 813.) Ordovicic. 

 Small, obtusely elliptical, acutely conical, apex slightly excen- 



