264 



NORTH AMERICAN INDEX FOSSILS. 



both valves of slight and nearly equal elevation. Surface faintly 

 striated. 



Helderbergian of New York, etc. 

 196. R. alsa Hall. (Fig. 318.) Devonic. 



Large, nearly transversely oval but hinge line somewhat ex- 

 tended, producing for some distance a straight posterior border. 



Fig. 318. Rhipidomella alsa and internal molds. (After Hall.) 



Pedicle valve depressed convex, becoming nearly flat. Brachial 

 valve convex and marked by a mesial sinus. Differs from R. 

 oblala in the absence of the depression in the pedicle valve, in the 

 more extended hinge line and in the presence of a sinus in the 

 brachial valve. 



Schoharie of New York, Michigan, etc. 



197. R. livia (Billings). (Fig. 319, a-b.) Devonic. 



Differs from R. vanuxemi in the lesser convexity of the brachial 



Fig. 319. a-b, Rhipidomella livia, c, R. michelini (after Hall) ; d-g, R. dubia. 

 (After Whitfield.) 



valve and in the absence of a mesial sinus, while the beak is shorter, 

 not rising to nearly the same height as in the pedicle valve. Its 

 suborbicular form distinguishes it from R. lencosia. 

 Onondaga of New York, Ohio, Ontario, Quebec. 



