PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 397 
Horizon and locality —“Ottosee”’ = Benbolt? in Virginia: Fugates Hill, Bris- 
tol (30’) Quadrangle. Trenton limestone in New York: On Dry Sugar River, 
Lewis County. 
DINORTHIS SWEENEYI (N. H. Winchell) 
Plate 57, B, figures 12, 13; plate 65, C, figures 13-17 
Illustrations of this species have been introduced to show the presence of a 
pseudodeltidium in the genus and also to show the variability of the ribbing. 
The occurrence of the pseudodeltidium is sporadic but is well known in the 
Minnesota species. Of Appalachian species only D. willardi shows this structure. 
Many of the species of the Appalachians do show intercalated costae along the 
margins, particularly in the vicinity of the posterolateral extremities. Dinorthis 
sweeneyi is usually without intercalated ribs, but a few specimens, such as the 
old adult figured herein (pl. 65), show them. 
Figured specimens.—48861, 118020a. 
Horizon and locality—Decorah formation (Ion member) in Minnesota: At 
St. Paul; and Cannon Falls, Goodhue County. 
DINORTHIS TENUIS Cooper, new species 
Plate 72, A, figures 1-5 
Shell fairly large for the genus, wider than long ; hinge narrow; sides rounded ; 
anterior margin broadly rounded; anterior commissure rectimarginate; surface 
costate, costae numbering about 38, undivided and primary. 
Pedicle valve with unevenly convex lateral profile, the umbonal region gently 
convex but the valve anterior to the umbo moderately concave ; maximum con- 
cavity in the anterior half; anterior profile broadly and gently concave. Umbonal 
region gently swollen; beak small; sides somewhat flattened. Interarea short, 
nearly catacline. 
Brachial valve moderately convex in lateral profile, broadly and gently con- 
vex in anterior profile; median region somewhat swollen ; lateral slopes long and 
gentle; anterior half marked medianly by a shallow, narrow, and poorly defined 
sulcus. 
Measurements in mm.—Holotype, length 16.2, brachial length 16.4, width 
24.6, thickness 4.7. 
Type—Holotype: 116974. 
Horizon and locality—Sevier formation in Tennessee: On the east side of 
the road, 2.7 miles S. 7° W. of Athens, Athens (T.VA. 125-NE) Quadrangle ; 
basal 2 feet of argillaceous nodular limestone 100 feet above the Meadow marble 
in a glade 3 mile N. 10° E. of Meadow Station, Meadow (T.V.A. 139-NW) 
Quadrangle ; Marysville road, 2 miles southeast of Friendsville, Binfield (T.V.A. 
139-NE) Quadrangle; cobbly layer in Sevier, $ mile north of Meadow, Meadow 
(T.V.A. 139-NW) Quadrangle. 
Tellico formation in Tennessee: 1.6 miles south-southwest of Toqua School, 
Vonore (T.V.A. 139-SW) Quadrangle. 
