988 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Hypotype (117323)........- 20.8 19.6 23.1 13.0 12.0 
€ (1a7gzna). piatick 18.7 17.4 10.5 12.5 II.0 
‘ (E27924b rents stor 16.8 15.4 18.0 11.7 0.1 
ce (48683 nesses 0.9 9.1 10.7 7.5 6.3 
Types.—Figured hypotypes: 48683, 117323, measured hypotypes: 117324a,b. 
Horizon and locality—Decorah formation (lon member—Phylloporina and 
Fucoid beds) in Minnesota: At St. Paul; Kenyon, Goodhue County; Fort 
Snelling, Hennepin County ; Milton, Dodge County ; cut 1 mile east, 1 mile north, 
and 4 mile east of Wanamingo, Goodhue County; Wagner Hill, 5 miles south 
of Cannon Falls, Goodhue County ; cut on the county road 1.1 miles east of U. S. 
Highway 52 on the south edge of Cannon Falls, Goodhue County; Elmira, 
Olmstead County. 
Decorah formation (Ion member) on the Platteville road in Wisconsin: 
3.5 miles southeast of Lancaster, Lancaster (30’) Quadrangle ; Wisconsin High- 
way 10, + mile west of Ellsworth, Pierre County. 
Decorah formation (Ion member-Phylloporina bed) in Iowa: At Decorah, 
Winneshiek County. 
Discussion —This species is characterized by its nearly circular form (L/W 
usually 0.90 or greater), narrow hinge, strongly rounded contours, and generally 
fine costellae. In size this species is like P. minnesotensis and P. subaequata 
but differs markedly from both of them. The wide hinge and shouldered out- 
line of P. minnesotensis are features which separate that species readily from 
P, circularis. 
Some specimens of P. subaequata have an outline like that of P. circularis and 
have almost the same length/width index, but the former species is much more 
strongly costellated than P. circularis. Furthermore, P. subaequata generally 
has a strong concentration of swollen costellae in the median region. 
PIONODEMA CONRADI (N. H. Winchell) 
Plate 154, C, figures 12-14; plate 162, A, figures 1-6 
Orthis conradi N. H. Wrncue tt, 8th Ann. Rep. Geol. Nat. Hist. Surv. Minnesota, p. 68, 
1880. 
Orthis subaequata Hatt (not Conrad), Pal. New York, vol. 1, p. 118, pl. 32, figs. 2e, f (not 
2a-d), 1843. 
Orthis (Dalmanella) subaequata conradi (N. H. Winchell) WiNcHELL and SCHUCHERT, 
Geol. Minnesota, vol. 3, p. 449, pl. 33, figs. 37-39, 1895. 
Pianodema subaequata conradi (N. H. Winchell) Basster, U. S. Nat. Mus. Bull. 92, p. 978, 
IQI5. 
Shell small for the genus, subcircular in outline ; hinge narrow, equal to about 
two-thirds the width; sides and anterior margin rounded; anterior commissure 
varying from rectimarginate to slightly uniplicate. Costellae of unequal size, 
crowded, numbering 2 to 5 to the millimeter at the front margin; numerous cos- 
tellae swollen and usually separated from each other by a very fine intercalated 
costella at the front margin. 
