PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 795 
lateral margins generally straight in outline and sloping moderately toward the 
middle. Anterior margin usually broadly rounded, occasionally slightly nasute 
when the fold is well developed. Posterior margin nearly straight. Surface multi- 
costellate ; costellae appearing in 3 generations. Second and first generation cos- 
tellae of about same size at front of valve; 1 to 2 of the finer costellae of the second 
and third generation alternate with the coarser ones. At the front margin of a 
fully adult valve about 120 to 130 costellae of all sizes may be counted, about 5 in 
I mm. at the front margin near the center. Between the costellae the shell is 
marked by fine, crowded concentric fila; in very well preserved specimens the 
costellae bear short imbricating spines. A few wrinkles along the posterior mar- 
gin occasionally seen. 
Pedicle valve moderately but unevenly convex in lateral profile; anterior pro- 
file with the greatest convexity in the middle but not greatly elevated and with 
the flanks sloping gently to the margins. Beak inconspicuous, protruding very 
slightly beyond the posterior margin. Umbo narrow, slightly swollen, extended 
forward as a low and narrow fold nearly to the front margin. Flanks bounding 
fold full in anterolateral regions; slopes to cardinal extremities moderate. A 
tendency to form an obscure plication along the middle of each flank has been 
observed. Interarea curved, strongly apsacline. Pseudodeltidium much reduced. 
Brachial valve with moderately concave umbo and with concavity extended 
forward as a narrow sulcus to the anterior margin or disappearing slightly be- 
fore the margin. Flanks bounding sulcus moderately concave or nearly flat. 
Cardinal extremities flattened and deflected slightly toward the pedicle valve. 
Chilidium deeply cleft to form chilidial plates. Interarea hypercline. 
Interior of pedicle valve with small teeth but strongly reenforced fossettes ; 
dental plates obscured by secondary deposits; apex of delthyrial chamber filled 
by callus deposit into which the adductor impressions are deeply sunk. Adjustor- 
diductor fields not widely divergent, subflabellate. Median ridge low and slender, 
extending about one-third the length. Pallial marks well impressed. Anterior 
and anterolateral portions strongly thickened and papillose. Brachial valve usu- 
ally without a greatly thickened periphery; median ridge low and inconspicu- 
ous; lateral ridges low, strongly elevated anteriorly ; muscular areas not greatly 
thickened. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
THolotyneae ays hick 8.1 7.0 11.9 14.3 1.9 2.6 
Paratype (117525c):..<. Out 8.4 12.8 15.2 aK 3.0 
a (1175234)... 7.8 7.1 11.9 14.0 1.8 2.3 
og (107521)es 7.9 TB II.0 12.4 2.1 2.7 
i: (117530a)... 9.0 8.6 12.6 13.6 1.9 213 
¢ (117530b)... 7.9 7.4 I1.2 13.5 Might 1.7 
if (117526a)... 8.9 8.4 132 14.4 1.9 2.6 
% (98206b).... 10.2 0.6 15.0 18.4 2.0 3.0 
Types.—Holotype: 117525a; figured paratypes: 98206a-d (Butts), 98207b, 
117522, 117523a, 117524a, 117525b, 117526a,b, 117527a,c, 117530a,b; unfigured 
