74 
IOWA ACADEMY OF SCIENCE Vor,. XXIX, 1922 
arises about midway of the valve ; is broad and flattened, bearing 
four plications. Postero-lateral slopes with slight vertical flat- 
tening. Surface of both valves marked by fourteen to eighteen 
strong, simple, subangular or rounded plications; growth lines 
lacking in all specimens examined. 
This species resembles, in general form, both R. minnesotense 
and R. wisconsinense. From the former it differs in its lesser 
width and greater thickness, fewer plications, and lesser eleva- 
tion of the fold. From the latter it may be distinguished by its 
lesser width, greater thickness, fewer and less angular plications, 
and complete lack of lamellose growth-lines. Its nearest relative 
seems to be R. cuneiforme of this paper, from which it is dis- 
tinguished by shape, pronounced curvature of the pedicle beak, 
and more rounded plications. 
Formations and Localities : Decorah, — 2 miles west of Ste. 
Genevieve, Missouri; Platteville, — Minneapolis, Minnesota; (?) 
Plattin, — Ste. Genevieve County, Missouri. . 
Cotypes, No. 27,443, Walker Museum. 
RHYNCHOTRBMA UNIPLICATUM sp. nov. 
Plate II, figs. 10-13; cf. figs. 14-16. 
Shell small, subtriangular in outline, wider than long ; postero- 
lateral margins straight, meeting at the beak in an acute angle ; 
anterior margin almost straight. Dimensions of the cotypes : 
Length, 11.8 mm., and 10 mm.; width, 12 mm., and 10.4 mm.; 
thickness, 7.7 mm., and 5.8 mm. ; width of sinus, 6.7 mm., and 
5.2 mm. 
Pedicle valve slightly convex in the umbonal region ; lateral 
slopes flattened. Mesial sinus sharply defined ; originates about 
3 mm. from the beak, and contains one strong plication. Brachial 
valve more convex than the pedicle. Mesial fold, originating 
about 3 mm. anterior to the beak, is low but well defined, and 
bears two strong, rounded plications. Beak sharply incurved, 
projecting into the delthyrium of the produced pedicle beak. 
Surface of each valve marked by eleven to fourteen strong, 
subangular or rounded plications, and by heavy, rounded growth- 
lines, so very coarse as to give a nodose appearance to the pli- 
cations. 
This species differs from R. kentuckiense varians of this paper, 
with which it is associated, in its lesser number .of • plications on the 
fold and sinus, and in the heavy growth lines which are not 
