618 THE PALEONTOLOGY OF MINNESOTA. 
[Rhytimya. 
gently arcuate. Beaks small, incurved, projecting but little above the hinge, situated 
about one-fifth of the entire length from the anterior extremity; umbonal ridge 
rounded, not strong, distinguishable chiefly in the upper third of the shell, where it 
causes a flattening or slight concavity in the slope to the cardinal edge. Sides of 
valves moderately convex, with point of greatest convexity a little in front of and 
above the middle. 
Casts of the interior exhibit a strongly defined ovate anterior muscular scar, 
bounded upon the inner side by a distinct linear depression which must have been 
produced by an internal ridge in the valves extending downward from the hinge 
just in front of the beaks. Immediately above the adductor impressions there is a 
minute but distinct pair of, presumably, pedal muscle scars. Posterior adductor 
impressions very faint, rounded, situated a short distance within the middle of the 
post-cardinal margin. Pallial line not very well defined, simple. Dorsum of cast 
exhibiting impressions of cardinal and lateral teeth, indicating a hinge as shown in 
figs. 13 and 14 on plate XLII. 
So far as known the beds from which this species was obtained contain no 
lamellibranch with which it is at all likely to be confounded. 
Formation and locality —Galena shales near Cannon Falls, Minnesota. 
Family PHOLADELLID, Miller. 
Genus RHYTIMYA, Ulrich. 
Orthodesma, WHITFIELD, 1878. Jour. Cin. Soc. Nat. Hist., vol. i, p. 139; MrEr, 1881, idem., vol. iv, 
p. 76. (Not Orthodesma, HALL and WHITFIELD, 1875, Pal. Ohio, 
vol. ii, p. 93.) 
? Sedgwickia, WHITFIELD, 1878. Jour. Cin. Soc. Nat. Hist., vol. i, p. 140. (Not Sedgwickia, McCoy 
1844, Synop. Carb. Koss. Ireland, p. 61.) 
Shell elongate, moderately ventricose, the dorsal and ventral margins subparallel, 
gaping slightly at one or both ends. Beaks rather prominent, situated from one- 
third to one-fifth of the entire length behind the anterior extremity; posterior 
umbonal ridge rounded, never very prominent; mesial sulcus wide, generally very 
shallow, often however causing a sinuosity in the ventral margin. Lunule very 
narrow, true escutcheon wanting, ligament external, attached to the edges of the 
valves, extending the greater part of the hinge line posterior to the beaks. Hinge 
apparently edentulous, test very thin. Muscular and pallial attachments exceedingly 
faint, not satisfactorily observed; posterior scar large. Surface marked with 
unequal concentric lines and furrows, gathered into a series of strong folds on the 
anterior end. On the posterior half or more, the ventral part especially, the concentric 
lines are crossed by closely arranged radiating series of small granules or spines. 
Type: Rhytimya producta n. sp. 
