374 NORTH AMERICAN INDEX FOSSILS. 



3 S. pauper Billings. (Fig. 1674, c.) Cambric. 



With strongly incurved posterior portion, and arched dorsum 

 with few coarse corrugations. 



Lower Cambric (Etcheminian) of Conception Bay, Newfound- 

 land and eastern Massachusetts. 



4. S. levis Walcott. (Fig. 1674, /.) Cambric. 



Elongate, stout, curved through less than a right angle; corru- 

 gations irregular, few (about three) of unequal strength, strongest 

 on ventral side. 



Lower Cambric (Etcheminian) of Conception Bay, Newfound- 

 land and boulders from eastern Massachusetts. 



IL RiBEiRiA Sharpe. 

 Arched, univalve shells, without corrugations, with strong beaks, 

 and open at the ends and along the basal margin; a thick, trans- 

 verse internal plate marks the anterior extremity behind which 

 is a corrugated boss for the attachment of muscles. Ordovicic. 



Fig. 1675. Ribeiria calcifera, left, anterior and dorsal views of carapace. 

 (After Billings.) 



5. R. calcifera Billings. (Fig. 1675.) Ordovicic. 

 Ovate, compressed, narrowed towards the posterior extremity; 



anterior end broadly rounded ; ventral margin curving its entire 

 length ; dorsal margin straight behind the beak ; a little concave in 

 front ; beaks from one fifth to one sixth distance from anterior end. 

 Beekmantownian of Canada and Pennsylvania. 



6. R. compressa Whitfield. Ordovicic. 

 Strongly compressed; valves nearly flat; about three fifths as 



high as long; both ends gaping; on internal mold is a strongly pro- 

 jecting beak, beneath which is a deep notch; muscular scar narrow, 

 on rounded dorsal edge. 



Beekmantownian (Fort Cassin) of Vermont, etc. 



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