402 



NORTH AMERICAN INDEX FOSSILS. 



97. L. parva (Rogers). (Fig. 512, c.) Eocenic. 



Minute (length about \ inch). Sharp oblique sulcus and sharp 

 concentric lines present. 

 Atlantic and Gulf regions. 



98. L. eborea Conrad. (Fig. 513.) Eocenic. 

 Distinguished by its almost triangular and 

 equilateral form. Surface smooth and pol- 

 ished. Length usually about \ inch. 

 Abundant in Midwayan of Gulf region. 



Fig. 513. Leda eborea 

 ( After Harris. ) 



XXX. Yoldia Moller. 



Compressed, lanceolate, gaping behind, and more or less pro- 

 longed and tapering posteriorly. Beaks turned backward. Sur- 

 face nearly smooth. Hinge as in Nucula. Pallial sinus deep. 

 Differs chiefly from Leda in the gaping valves and in the deep pal- 

 lial sinus. Cretacic-Recent. 



99. Y. septariana Cragin. Cretacic. 

 Elongate, subovate, with beaks anterior to middle. Hinge line 



sloping abruptly on both sides from the beaks. Teeth 16-20 in 

 front of the beaks and 21-29 behind. Surface marked with con- 

 centric lines. Length -| inch ; height -| inch. 



Very abundant in septaria of the Benton (Eagle Ford) of Texas. 



100. Y. evansi Meek and Hayden. (Figs. 514-515.) Cretacic. 



Fig. 514. Yoldia evansi, internal 

 mold showing pallial sinus and 

 hinge-denticles. (After Meek.) 



Fig. 515. Yoldia evansi (upper) 

 left valve; Y. scitula (lower), 

 left valve. (After Meek. ) 



Outline similar to that of Y. longifrons but the shell is smaller 

 (length not exceeding -| inch), and the surface is more nearly 

 smooth, the concentric striae being very obscure. 



Fox Hills : South Dakota, Montana, Assiniboia. Pierre : Mon- 

 tana (Claggett), North Dakota (Bearpaw). Also Wyoming. 



