298 NORTH AMERICAN INDEX FOSSILS. 



283. R. hubbardi A. Winchell. Mississippic. 

 Small. Cardinal slopes straight. Greatest width about the 



middle of the shell. The two valves equally convex. Front 

 gently rounded. Pedicle valve with a broad, 

 shallow sinus occupied by 8 or 9 plications. 

 Brachial valve with no fold perceptible but 

 with a shallow sinus extending from the beak 

 for about one third the length of the shell. 

 Fig. 370. Rhyncho- Surface COV ered with about 21 narrow pli- 



nella eurekaensis. (Af- 

 terWalcott.) CatlOUS. 



Waverly of Ohio, Marshall of Michigan. 



284. R. myrina Hall and Whitfield. Jurassic. 

 Of medium size, broadly ovate with greatest diameter below the 



middle. Brachial valve the deeper, nearly evenly convex from 

 beak to front and with a slightly elevated median fold. Pedicle 

 valve with a prominent projecting beak and a broad, moderately 

 depressed median sinus. Surface marked by 31-34 low, rounded 

 plications, 8-10 of which occupy the fold and sinus. 

 Dakota, Utah. 



285. R. aequiplicata Gabb. Triassic. 

 Subglobose. Valves equally convex. Beak large, prominent 



and incurved. Sides sloping convexly to extremities of hinge 

 line. Surface with about 14 nearly equal, rounded ribs, beginning 

 on each valve about one third the distance from beak to front ; the 

 furrows equalling the ribs in width. Length and width about 1 

 inch each but length slightly exceeding width. Growth lines 

 faintly developed. 



Trias of Humbolt Mt., Nevada. 



286. R. gnathophora Meek. Jurassic. 

 Of medium size, subtrigonal, with greatest convexity near the 



middle. Postero-lateral margins straight, diverging from beak 

 at an angle of 8o° to ioo°. Brachial valve usually the more con- 

 vex, especially near the front where it rises into a median fold 

 traceable for about half the way toward the incurved beak. Pedicle 

 valve with a corresponding sinus; beak pointed. Surface with 16 

 to 20 simple plications of which three or four are in the sinus and 

 four or five on the fold. Length about .8 inch and breadth about 

 .7 inch. 



Jurassic of California and Utah. 



