348 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
ing a moderately deep depression in the median and anteromedian regions. 
Flanks bounding sulcus flat ; posterolateral regions deflected slightly toward the 
pedicle valve; chilidium well developed, short; cardinal process stout; brachio- 
phores stout, moderately long; median ridge low and long. 
Measurements in mm.—Holotype, length 14.3, brachial length 11.4, width 14.9, 
hinge width 14.6, thickness 6.9. 
Type.—Holotype: 48796. 
Horizon and locality—Decorah formation (Ion member) in Goodhue County, 
Minnesota: 8 miles south of Cannon Falls; in a road cut on the county highway 
1.1 miles east of U. S. Highway 52 on the south edge of Cannon Falls; Wagner 
Hill, U. S. Highway 52, 5 miles south of Cannon Falls. 
Same formation and member in Iowa: Along the Dugway, 4 mile west of 
Decorah. 
Discussion.—This species is characterized by having the length and width 
almost equal, a long interarea, and somewhat crowded costae. The species is also 
characterized by the unusual development of the pseudodeltidium and chilidium. 
In most species of Hesperorthis the pseudodeltidium, when present, is short and 
located at the posterior end of the delthyrium. In this species a pseudodeltidium 
measuring 4 mm, in length in a delthyrium 6.5 mm. in length has been observed. 
Furthermore, the pseudodeltidium is often considerably thickened and in at least 
three cases is swollen above the level of the interarea. A chilidium is an unusual 
feature in this genus, but most of the specimens of this species examined possess 
it. The chilidium, however, is generally short but is well arched. 
Another feature of interest is the low ridge along the margin of the lateral 
plates bounding the delthyrium. The species differs from H. tricenaria (Conrad) 
in the excessive development of the pseudodeltidium, smaller size, more concave 
brachial valve, and more strongly apsacline interarea on the pedicle valve. 
HESPERORTHIS CONCAVA Cooper, new species 
Plate 51, E, figures 21-26; plate 54, C, figures I1-15 
Shell of medium size for the genus, length and width about equal ; cardinal ex- 
tremities approximately a right angle or slightly obtuse; sides gently rounded ; 
anterior margin narrowly rounded; anterior commissure rectimarginate; sur- 
face marked by about 30 narrowly rounded, strong costae separated by spaces 
slightly narrower than the costae. Fine costellae mark troughs of the striae. 
Pedicle valve strongly convex in lateral profile with the maximum convexity 
slightly posterior to the middle; anterior profile strongly but broadly convex and 
without the median narrowing characteristic of many species of this genus; 
lateral slopes gently convex, steep; beak somewhat elongated, obtuse, forming 
an angle of about 130°; median region strongly swollen with steep anterior, 
lateral, and posterolateral slopes. Fold formed by an elevated median costa ex- 
tending from beak to anterior margin but most prominent in anterior two-thirds. 
Interarea long, fairly strongly curved, strongly apsacline; pseudodeltidium 
short. 
