454 



NORTH AMERICAN INDEX FOSSILS. 



line. Anterior margins of valves abruptly inflected from the um- 

 bonal ridge so as to meet on a plane at right angles to that of the 

 valves. Beaks attenuate, compressed in front 

 and behind. Posterior extremity of hinge 

 angular. 

 Chester of Illinois. Kaskaskia of Missouri. 



248. M. congeneris Walcott. (Fig. 601.) 

 Mississippic-Carbonic. 



Differs from M. perattennata in having the 

 anterior and posterior margins parallel ; also- 

 the anterior margin is straight while in M. 

 perattennata it is concave. 



Mississippic : Nevada. Upper Coal Meas- 

 ures of Kansas. 



249. M. swallovi McChesney. (Fig. 602, 

 a, b.) Carbonic. 



Small, nearly equivalve. Cardinal area 

 very narrow. [Doctor Hind makes this a synonym of the Eng- 

 lish shell Naiadites triangularis Sowerby.] 



Indiana-Wyoming 

 250. M. recurvirostris Meek and Worthen. (Fig. 602, c.) 



Carbonic. 

 Convex. Beaks pointed ; that of left valve twisted. Anterior 



Fig. 601. Myalina 

 congeneris. (Kan. Pal. 

 VI/II.) 



FlG. 602. a, b, Myalina szuallovi, dorsal view and right valve ; c, M. recurvirostris r 

 interior of left valve. (Ind. Survey. ) 



margin thickened beneath the beaks so as to present a kind of 

 false area. Indiana-Iowa. 



