554 NORTH AMERICAN INDEX FOSSILS. 



feet ; diameter at top 10 inches ; thickness of outer layer of shell 

 3 inches. 



Niobrara : Kansas. 



CLII. Tancredia Lycett. 

 Trigonal, usually with nearly central beaks, but anterior side 

 attenuated and somewhat the longer and posterior wider, shorter, 

 and usually obliquely truncate. Somewhat gaping posteriorly. 

 Margins entire. Surface smooth. One cardinal and one posterior 

 lateral tooth in each valve. Ligament external. Pallial line 

 simple. Triassic-Cretacic. 



528. T. bulbosa Whitfield. (Fig. 759.) Jurassic. 



Anterior portion marked with a broad and 

 shallow depression. Surface smooth except for 

 very fine concentric lines. 



Sundance : South Dakota, Wyoming. 



529. T. americana Meek and Hayden. 



Fig. 759- Tan- Cretacic. 



( After Loean ') Thick. Posterior end broader than anterior, 



obliquely truncate and gaping above. Anterior 



narrow and somewhat rostrated. Hinge line slightly concave 



anterior to beaks and convex posteriorly. Beaks small. Length 



2.5 inches. 



Mohtanan : Colorado, Alberta, Assiniboia. 



CLIII. Paracyclas Hall. 

 Suborbicular, thin-shelled, with small and low beaks and short 

 hinge line. Posterior portion near the beaks more or less defined 

 by an oblique furrow, sometimes with posterior hinge extremity 

 almost winged. Surface marked with concentric striae which are 

 often developed into ridges. No lunule present. Ligament set in 

 a deep groove. Pallial line simple. Devonic. 



530. P. ohioensis (Meek). (Fig. 760, c.) Devonic. 

 Anterior end produced and flattened. Concentric striae very 



fine. Rather small. 



Onondaga : Ohio, Indiana. 



531. P. elliptica Hall. (Fig. 760, d.) Devonic. 

 Large. Concentric striae lamellose at irregular intervals. 



