620 THE PALEONTOLOGY OF MINNESOTA. 



[Cuneamya. 



ventral margin broadly sinuate in the middle, gently convex on each side of the 

 center; posterior margin very slightly oblique, strongly rounded; anterior end sub- 

 rectangular, most prominent about the middle of the hight, the upper half nearly a 

 straight slope to the beaks, the lower rounding backwards into the base. Beaks 

 strongly incurved, situated almost a third of the entire length from the anterior 

 extremity; umbones large and prominent, constricted by the mesial sulcus which 

 crosses the valves and produces the sinus in the basal line. Prom the strongly con- 

 vex posterior umbonal ridge the surface descends abruptly to the cardinal margin. 

 Lunule larger than usual, of moderate depth and definition. Surface markings 

 obscure on the cardinal slope of the cast studied, on the umbones and flanks, con- 

 sisting of somewhat irregular, shallow, concentric furrows and fine striae. On the 

 anterior end these markings are strengthened or gathered into about twelve strong 

 folds, terminating at the margin of the lunule, and increasing regularly with the 

 growth of the shell. In having a large lunule, comparatively long anterior end, and 

 unusually prominent umbones, this species approaches B. lunulata W bitfield, sp., and 

 reminds somewhat of Pholadella, Hall. The characters mentioned readily distinguish 

 the form from all the other species now referred to Ehytimya, Of associated shells 

 only Cuneamya truncatula has a concentrically furrowed surface but that species differs 

 too widely in other respects to be confused with R. sinuata. 



Formation and locality Middle Galena near Wykofl, Minnesota. 



Family GRAMMYSIID^E, Hall. 

 Genus CUNEAMYA, Hall and Whitfield. 



Cuneamya, HALL and WHITFIBLD, 1875. Pal. Ohio, vol. ii, p. 90. 



Thin, fragile, closed, bivalve shells, with ventricose valves and strong, prominent 

 incurved beaks, situated but little behind the anterior extremity; outline varying 

 from subcircular to somewhat elongate subrhomboidal; cardinal line very nearly 

 straight behind the beaks. Hinge linear, edentulous: valves probably held together 

 at the hinge solely by an external ligament. Cardinal margin of valves inflected, 

 forming a long escutcheon or false area posterior to the beaks; anteriorly a lunule, 

 varying considerable in depth and shape, but always well defined, is situated beneath 

 the beaks. Muscular and pallial impressions too faint to be determined with cer- 

 certainty. Surface marked by more or less distinct concentric plications or 

 wrinkles, which are usually rather obscure on the cardinal and posterior slopes and 

 always the most regular and distinct on the anterior side of the umbonal region. 

 Occasionally the surface is nearly smooth. An undefined, broad and shallow mesial 

 sulcus usually present. 



