PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 987 
cause of the presence of the low median septum extending anterior to the adduc- 
tor impression. The cardinal process has a thin but long shaft and is surmounted 
by a small lobate myophore like that of Pionodema. The brachial plates are 
flaring and like Pionodema, but they are extended anteriorly and inward to 
surround the anterior end of the cardinal process shaft and thus to form a long, 
partially enclosed chamber that simulates Doleroides. No other species of Piono- 
dema has been seen with such a structure, although some specimens of P. subae- 
quata approach it. This species is important because it occurs just under the 
Bays red beds and thus helps to date the red bed series and the Sevier under 
it. This Pionodema suggests an early Trenton age for the beds in which it occurs. 
PIONODEMA CIRCULARIS (N. H. Winchell) 
Plate 154, A, figures 1-5; plate 154, B, figures 6-11 
Orthis circularis N. H. WINCHELL, 8th Ann. Rep. Geol. Nat. Hist. Surv. Minnesota, p. 66, 
1880. 
Orthis (Dalmanella) subaequata circularis (N. H. Winchell) Wi1ncHELL and ScHUCHERT, 
Geol. Minnesota, vol. 3, p. 452, pl. 33, figs. 46, 47, 1895. 
Pianodema subaequata circularis (N. H. Winchell) Basster, U. S. Nat. Mus. Bull. 92, vol. 2, 
p. 978, 1915. 
Pionodema cf. subaequata (not Conrad) Cooper, Journ. Paleont., vol. 4, No. 4, pl. 36, fig. 5, 
1930. 
Shell large for the genus, subcircular in outline; hinge narrow, slightly less 
than two-thirds the width. Cardinal extremities obtuse; sides and anterior mar- 
gin broadly rounded. Anterior commissure gently uniplicate. Very finely cos- 
tellate, costellae crowded along the front margin and numbering 3 to 4 per 
millimeter in the fold and sulcus region but 4 to 5 per millimeter on the flanks. 
Hollow costellae not strongly swollen, concentrated on fold and in sulcus but 
also scattered over most of the surface. 
Pedicle valve unevenly convex in lateral profile and with the maximum curva- 
ture near the middle; anterior third somewhat flattened ; anterior profile broadly 
and moderately convex, most so in the middle and with moderately steep, flat 
sides. Beak small and strongly incurved ; umbo swollen; median region swollen; 
anterior third depressed to form a broad and shallow sulcus; flanks somewhat 
flattened. Interarea curved, apsacline. 
Pedicle interior with prominent oval muscle field; diductor scars elongate, 
somewhat flabellate anteriorly ; adductor scars located on the sides of a promi- 
nent median ridge. 
Brachial valve moderately convex in lateral profile and with a strongly con- 
vex umbo; anterior profile moderately and fairly evenly convex and with slightly 
less depth than the pedicle valve. Umbo gently and narrowly sulcate, the sulcus 
disappearing about one-third the length from the beak; median region swollen ; 
lateral slopes moderately steep. Beak small, incurved. Median fold generally 
indistinct. Brachial interior with small cardinal process having a short shaft; 
brachiophore plates short and flaring. Median ridge low and inconspicuous. 
