388 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
broadly convex; beak prominent, pointed, median costa slightly depressed with 
adjacent costae to form a shallow sulcus; flanks slightly convex but the lateral 
slopes to the cardinal extremities steep. Delthyrium partially covered by a short. 
depressed, concave apical plate, possibly the seat of pedicle attachment. Muscle 
area subcordate in outline with prominent narrow, elongate adductor impres- 
sions, a conspicuous adductor field bordered by low ridges. Adjustor impres- 
sions small and located at the base of the dental plates. Teeth large, dental plates 
short and receding. Pallial impressions indistinct. 
Brachial valve strongly convex in lateral profile with the most convex part in 
the strongly arched umbonal region. Anterior profile bilobed. Sulcus originat- 
ing on the convex umbo, deep and narrow and extending to the anterior margin 
which is produced into a short, sharp tongue. The 2 costae bounding the sulcus 
are also depressed below the flanks which are narrowly rounded and have steep 
slopes to the margins. Interarea long, curved, orthocline to anacline. Sockets 
deep, brachiophores ponderous with strong cuplike thickenings of callus attach- 
ing them to the lateral walls. Notothyrial cavity deep and notothyrial platform 
thick, supporting a septumlike cardinal process much thickened at its anterior 
end. Median ridge strong and thick. 
Measurements in mm.— 
Hinge 
Length Width width Thickness 
Holotype (pedicle valve) .......... 3.9 6.2 5.0 2.5 
Paratype (brachial valve 110097b).. 4. 5.8 4.7 2.0 
Types.—Holotype: 110097a; figured paratypes: 110097b-d. 
Horizon and locality —Effna-Rich Valley formations in Virginia: Porterfield 
Quarry, 5 miles east of Saltville, Maccrady (T.V.A. 218-NW) Quadrangle. 
Discussion.—A structure rarely preserved in these shells appears in the speci- 
mens on which this species is based. This is the small apical plate at the apex of 
the delthyrium. This is slightly depressed below the margin of the delthyrium 
and is attached to the sloping face of the lateral plates bordering the delthyrium. 
This plate is unlike a true pseudodeltidium in its concave form and may have 
been the seat of pedicle attachment. The lateral plates, possibly homologous to 
deltidial plates, are fairly well developed in the specimens studied. 
For comparison see P. rectangulata. 
PTYCHOPLEURELLA OKLAHOMENSIS Cooper, new species 
Plate 42, G, figures 40-44 
Shell small, wider than long with the hinge slightly narrower than the greatest 
shell width which is at the middle. Sides narrowly rounded ; front margin broadly 
rounded. Surface costate, about 16 broadly angular costae with interspaces about 
equal in width to the width of the costae. Entire surface regularly imbricate with 
about 4 to 5 imbrications to a millimeter. 
Pedicle valve hemipyramidal with a very gently convex lateral profile but a 
broadly convex anterior profile. Beak and umbo smooth. Median 5 costae 
slightly elevated to form a broad indistinct fold. Slopes to cardinal extremities 
