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. From 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 striz. 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 R. lunulata Whitfield, sp., and 
reminds somewhat of Pholadella, Hall. The characters mentioned readily distinguish 
the form from all the other species now referred to Rhytimya. 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 Wykoff, Minnesota. 
Family GRAMMYSIIDA, Hall. 
Genus CUNEAMYA, Hall and Whitfield. 
Cuneamya, HALL and WHITFIELD, 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. 
