PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 805 
concavo-convex. Interareas as in Sowerbyella. Surface of both valves orna- 
mented by costellae of unequal size; a few stronger costellae stand out in a field 
of finer, closely crowded costellae which may be cancellated by concentric fila. 
Pseudodeltidium of pedicle valve well developed; chilidium highly arched. 
Inside the pedicle valve the muscle field is small and confined to the posterior 
quarter of the valve. Adjustor-diductor impressions small, widely divergent, not 
separated by a septum. Brachial cardinalia like Sowerbyella with cardinal proc- 
ess frozen to the brachiophore bases to form an inverted V. Brachial processes 
long and slender, nearly parallel to the posterior margin. Adductor field bilobed 
with the ends of the lobes thickened ; anterior and posterior patches of each lobe 
separated by a prominent septum pointing toward the anterolateral curve of the 
front margin. Median septum short, located at about the center and mostly con- 
fined to that part of the valve. Two pallial trunks originate at the end of the 
median septum. 
Genotype.—Antsopleurella tricostellata Cooper, new species. 
Discussion.—This is an uncommon genus now known chiefly from limestone 
associated with black shales. It occurs in Great Britain as the Upper Bala 
“Sowerbyella”’ quinquecostata McCoy. This interesting little shell may be identi- 
fied by its generally sowerbyelloid form, generally low convexity, and two sizes of 
costellae. The interior of the brachial valve is also distinctive in the rounded 
adductor areas on each side of a low and delicate median septum. These areas 
are usually oval in outline, the narrow part of the oval directed posteromedially 
and the expanded part facing the anterolateral extremity. The areas are bounded 
by a curved and elevated rim, and each is divided more or less evenly by a short 
oblique ridge. The cardinalia are like those of Sowerbyella, but the cardinal 
process was not seen as a vertical ridge although the tentlike structure is present. 
ANISOPLEURELLA INAEQUISTRIATA Cooper, new species 
Plate 193, B, figures 5-9 
Shell small, width between 24 and nearly 3 times the length. Cardinal extremi- 
ties acute, with small ears. Lateral margin strongly indented just anterior to the 
ears in young specimens but very slightly rounded and oblique in adults. An- 
terior margin broadly rounded to nearly straight. Commissure uniplicate except 
at the cardinal extremities. Surface multicostellate, marked by costellae of 3 
sizes. On the young and the more youthful adults the shell is divided into 
4 sectors of about 45° each by 3 strong costellae, 1 central one, and 2 lateral ones 
that extend to the anterolateral margins. In older adults the prominence of these 
3 costellae at the front of the valve is obscured by the presence of many costellae 
of intermediate size, but even in these older specimens the 3 major costellae stand 
out. The 2 stronger sets of costellae are set in a bed of very fine costellae about 
17 in 1 mm. This figure usually includes at least 2 costellae of intermediate size 
of which there are about 21 including the 3 primary costellae. All the costellae 
are cancellated by fine concentric fila. In addition to these, fine concentric undu- 
lations are present. 
Pedicle valve unevenly convex with the greatest convexity in the anterior half 
