LAMELLIBRANCHIATA. 383 



Valves regularly convex in the posterior portion, becoming very gibbous 

 and ventricose in the middle and umbonal region. 



Beaks sub-anterior, prominent and strongly incurved. Umbonal slope 

 sub-angular, extending to the post-inferior extremity, producing a slight 

 angularity in the margin. Post-cardinal slope broad, marked by a distinct 

 fold along the middle, which produces an undulation in the margin of the 

 shell. A shallow cincture extends from the beak, reaching the ventral 

 margin at about the anterior third. 



Surface marked by fine concentric striae, which are somewhat fasciculate 

 on the posterior portion of the shell, and by fine radiating pustulose striae in 

 well-preserved specimens. The anterior half of the shell is marked by 

 strong sub-angular folds, or undulations, which become obsolete on the mid- 

 dle of the shell, or sometimes extending to the umbonal angle. These con- 

 centric folds are frequently bifurcate at or about the cincture. Interior 

 essentially unknown. 



Two specimens measure respectively 36 and 39 mm. in length, and 22 and 

 22 mm. in height. 



This species more nearly resembles G. Hannibalensis than any other, but the 

 shell is more gibbous, the cincture is stronger, the posterior extremity very dis- 

 tinctly truncate and the post-cardinal slope with a more or less defined fold. 



Formation and locality. In the yellow sandstones at Burlington, Iowa. 



Grammysia (Sphenomya) cuneata. 



PLATE LXII, FIGS. 1-9 ; AND PLATE XCIII, FIG. 19. 



Orammysia (Sphenomya) cuneata. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 62, figs. 



1-9. 1883. 



Shell of small or medium size ; ovate-cuneate, very oblique ; length more than 

 one-half greater than the height ; basal margin regularly curved, with some- 

 times a visible constriction toward the posterior end. Posterior end nar- 

 rowly rounded at the extremity and obliquely truncate above. Cardinal 

 line short ; margins apparently not inflected. Escutcheon large, limited by 

 an indistinct groove and fold. Anterior end very short, abruptly rounded 

 below the lunule. 



