PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 873 
Pedicle valve unevenly convex in lateral profile, the posterior third gently 
convex, the anterior two-thirds nearly flat; anterior profile broadly but gently 
convex and with the median region somewhat humped; umbonal region and 
median third gently swollen to form a low but perceptible median fold, visible 
nearly to the front margin. Flanks long and gently sloping; anterior slope long 
and gentle; interarea moderately long, gently apsacline. Pseudodeltidium small 
and wide. Muscle field nearly circular, large; subperipheral rim narrow and 
elevated. 
Brachial valve nearly flat in lateral profile but with the posterior third flat and 
the anterior two-thirds gently convex; anterior profile broadly and slightly con- 
vex; umbo and median posterior third gently concave; flanks and anteromedian 
two-thirds flat or perceptibly convex. Interior unknown. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
Eigpotype. (1170078)... <»...» -- 41.7 ? 45.1 49.2 ? ? 
‘i (TI7607D) ee oe 36.6 35.4 43.5 ? 6.8 
e (TE7GOR) L OTEIO. 3 45.4 ? 50.3 47.0? ? 
Types.—Figured hypotypes: 117695c, 117696, 117697,b. 
Horizon and locality—Lincolnshire formation (Hogskin member) in Ten- 
nessee: At Lee Valley, Lee Valley (T.V.A. 171-NW) Quadrangle; Evans 
Ferry section, U. S. Highway 25E, north of Indian Creek, Howard Quarter 
(T.V.A. 162-NW) Quadrangle. 
Lincolnshire formation in Tennessee: 4 mile north and northwest of Eidson, 
Kyles Ford (170-SE) Quadrangle. 
Same formation in Virginia: At Clinchport, Clinchport (T.V.A. 188-NW) 
Quadrangle ; Speers Ferry, Clinchport (T.V.A. 188-NW) Quadrangle; on U. S. 
Highway 19, 0.3 mile east-southeast of Hansonville Post Office, Hansonville 
(T.V.A. 205-SW) Quadrangle; north of Copper Creek on the railroad between 
Speers Ferry and Clinchport, Clinchport (T.V.A. 188-NW Quadrangle. 
Discussion.—This is a large species characterized by its gentle curves in lateral 
profile, the swollen umbonal region, and moderately strong costellae. It is clos- 
est to R. tennesseensis but is readily distinguished by its depressed form and 
different proportions, the length and width being nearly equal. It is a larger 
species than R. subparallela but is not so strongly convex in the anterior part 
of the brachial valve as that species. 
RHIPIDOMENA sp. 1 
Plate 213, D, figures 23, 24; plate 248, B, figures 2, 3 
Three imperfect specimens are the only trace of this genus yet to be taken 
from the Arline formation. These indicate a subquadrate species of low con- 
vexity and concavity. The posterior half of the pedicle valve is convex and the 
anterior half very gently concave. The umbo is fairly strongly swollen, but the 
swelling dies down at the middle of the valve. The umbo of the brachial valve 
is marked by a narrow, shallow sulcus that extends to about the middle of the 
valve. The posterior third of the brachial valve is somewhat flattened, but the 
