PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER gI5 
Muscle area equal in length to about one-third the valve length, deeply bilobed 
at the front. 
Brachial valve nearly flat to gently convex in lateral profile; sides narrowly 
and abruptly bent toward the brachial valve. Interior with prominent, stout but 
short septa anterior to the adductor field; septa between adductors stout. 
Measurements in mm.— 
Brachial Hinge Thick- Height 
Length length Midwidth width ness 
Holotype (pedicle valve) .......... 19.6 iy ates 19.8 ? 7.3 
Paratype (brachial valve 117821b).. ? 15.6 18.2 19.4 ? ? 
ff Gis “ET TZSQa) le. coer 14.2 18.6 19.6 ? i 
Types.—Holotype: 117821e; figured paratype: 117821a; unfigured paratypes: 
117821b-d,f. 
Horizon and locality—Chatham Hill formation in Virginia: 1.2 miles $.31° 
KE. of Sharon Springs, Burkes Garden (15’) Quadrangle. 
Discussion.—The species is different from all other described species except 
one. It is characterized by an unusually flat posterior region and a strongly 
and abruptly geniculated anterior, which produces a long trail in the pedicle 
valve. The only other species suggestive of this one is O. ? dubia, but that is 
smaller and has a shorter geniculated region. Interiors of the brachial valve 
have the important elements of Opikina. 
OPIKINA PARVULA Cooper, new species 
Plate 244, A, figures 1-9 
Small for the genus, subrectangular in outline, wider than long; hinge nar- 
rower than the midwidth; sides subparallel; anterior margin broadly rounded; 
4 costellae in I mm. at the anterior margin; posterior margin with short, oblique 
wrinkles. 
Pedicle valve geniculated anterior to the middle in lateral profile; angle of 
geniculation about 115°; anterior profile strongly domed; geniculation occur- 
ring at 6 to 8 mm. anterior to the beak; geniculated part narrowly rounded; 
visceral region gently convex in lateral profile; anterior slope convex and steep. 
Lateral slopes steep. Posterior margin nearly straight; beak small and slightly 
protuberant. 
Brachial valve with visceral region gently concave; umbonal region concave, 
the concavity forming an indistinct sulcus to about the middle; sides moderately, 
and anterior strongly, deflected toward the brachial valve to surround the visceral 
concave region. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
EVIE jis sie iniess 5.6 0 ae 11.7 10.8 13.5 127 27 4.8 
Paratype (117823b)..... 10.2 9.2 pee, 10.2 2.4 4.2 
(417822a))ce os 9.8 ? 8.8 ? EL. 12.3 2.0 3.6 
Types.—Holotype: 117823a; figured paratype: 117822a; unfigured paratypes: 
117822b,c, 117823b,c. 
