70 
IOWA ACADEMY OF SCIENCE Vot. XXIX, 1922 
lateral slopes flattened vertically, meeting a similar flattened space 
on the opposite valve, the whole forming a very pronounced, 
vertical area in which the plications are either greatly reduced or 
are lacking. In the young specimens this area is very much 
more pronounced than in the adults, in which there is a tendency 
to develop a convexity of the lateral slopes and a curvature of the 
postero-lateral margin. Mesial fold, which begins at the beak 
as a slight depression, becomes elevated at about 4 mm. from 
the beak; in the young it is low and flat; in the mature, very 
prominent and rounded. Surface of both valves marked by 
eleven to seventeen strong, acutely angular plications, those near 
the fold and sinus being extremely high and sharp. The three 
(in one case, four) plications of the sinus, and four of the fold 
are finer than those of the slopes. Anterior slopes of both valves 
much more abrupt in the young than in the adults. 
The resemblances and differences between this species and 
R. kentuckiense, with which it appears to be associated, are 
discussed in connection with that species. They are, as may be 
seen by a comparison of the figures showing the young of both, 
more pronounced among the young than among the adults. 
Formations and Localities : Black River, Mercer County, Ky. ; 
Curdsville (?), Curdsville, Kentucky. 
Cotypes, No. 27,449, Walker Museum. 
RHYNCHOTRRMA MINNES O T BN SB (SARD.) 
Plate I, figs. 15-17. 
1891. Rhynchonella Minnesotensis Sardeson, Bull. Minn. Acad. 
Nat. Sci., 3, p. 333, pi. 4, figs. 21-23. 
1893. Rhynchotrema inequivalvis Winchell and Schuchert (part), 
Geol. Minn., 3, pt. 1, p. 459, pi. 34, figs. 9-11 ; 15-23. 
Shell small, broadly subovate, wider than long; postero-lateral 
margin curved, without flattened area. Dimensions of two ma- 
ture specimens and one young: Length, 10.8 mm., 10 mm., and 
7.3 mm.; width 12.6 mm., 11.4 mm., and 7.9 mm.; thickness 8.6 
mm., 7.9 mm., and 3.3 mm.; width of sinus 7.1 mm., 5.7 mm., 
and mm. 
Pedicle valve uniformly convex in the young ; moderately so 
in the adults, with a considerable flattening anterior to the umbo. 
Beak sharp, slightly incurved, produced beyond that of the oppo- 
site valve; sinus originates 4 to 5 mm. from the beak and con- 
