618 THE PALEONTOLOGY OF MINNESOTA. 



[Rhytbnya. 



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^E, Miller. 

 Genus RHYTIMYA, Ulrich. 



Orthodesma, WIHTFIELD, 1878. Jour. Oin. Soc. Nat. Hist., vol. i, p. 139; MILLER, 1881, idem., vol. iv, 



p. 76. (Not Orthodesma, HALL and WHITFIELD, 1875, Pal. Ohio, 

 vol. ii, p. 93.) 



? Sedgwickia, WHITFIELD, 1878. Jour. Gin. Soc. Nat. Hist., vol. i, p. 140. (Not Sedgwickia, MoCoY 



1844, Synop. Carb. Foss. 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 scries of small granules or spines. 



Type: Rhytimya produda n. sp. 



