786 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Pedicle valve unevenly convex in lateral profile with the maximum convexity 
just anterior to the umbo; anterior profile with the median region narrowly ele- 
vated to form a moderately elevated hump with the sides concave and descending 
moderately to the margins; beak small; umbo somewhat narrowly swollen and 
passing into the moderately inflated median region; umbonal slopes to posterior 
margin short and moderately steep. Anterior slope long and gentle; posterior 
slope short and steep. Interarea moderately long, strongly apsacline; pseudo- 
deltidium small. 
Brachial valve most concave just anterior to the umbo which is also concave; 
lateral areas gently concave and posterolateral extremities somewhat flattened. 
Pedicle interior with elongate diductor scars diverging at an angle of about 
55°; median ridge low, anterior forks of median ridge low; pallial trunks not 
strongly developed. Brachial valve with median ridges low and closely spaced ; 
adductor callosity elongate, narrowly oval; marginal thickening not strong. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
IHOLOLY DE: ponisrae see sees 9.6 9.2 T52 16.9 1.8 2.5 
Paratype (117499a).. 9.1 8.7 13.2 15.1 £7 2.2 
a (117500a).. 11.9 11.2 17.0 19.4 ? e 
i“ (117500c).. 9.8 0.5 13.5 15.5 2.1 2.6 
“ (117500d).. 11.4 II.0 15.5 16.1 ? 2.3 3.2 
- (117500e).. 9.2 8.9 14.1 16.2 1.9 2.8 
“ (117500f).. 9.6 9.0 15.1 16.5 1.3 2.3 
4 (1175001) .. ? 8.8 13.8 14.8 ? ? 
5 (117500-1) . ? 8.9 12.0 12:3 ? ? 
. (117500m) . 11.2 10.5 16.6 17.6 2.8 3.6 
Types —Holotype: 117500b; figured paratypes: 117499a,b, 117500a,c,g,i; un- 
figured paratypes: 117500d-f,h,j-m. 
Horizon and locality—Decorah formation (Ion member) in Goodhue County, 
Minn.: about 1 mile east of U. S. Highway 52, on the south side of Cannon 
Falls; Wagner Hill, on U. S. Highway 52, 5 miles south of Cannon Falls; 
Kenyon; Minneapolis; and in Fillmore County: On U. S. Highway 52, 1 mile 
south of Fountain. 
Same formation in Iowa: Along the Dugway, 4 mile west of Decorah, Winne- 
shiek County. 
Same formation in Wisconsin: From a cut on Wisconsin Highway Io, 4 mile 
west of Ellsworth, Lancaster (30’) Quadrangle. 
Discussion.—Its large size and its well-differentiated and strongly beaded cos- 
tellae make this species comparable with S. punctostriata and similar species. 
The Minnesota species differs from S. punctostriata in its lesser width and more 
compact form. It is a much less robust form than S. curdsvillensis, and its in- 
teriors are not so strongly thickened. Sowerbyella eximia is somewhat differ- 
ently proportioned and has a completely different ornamentation. 
