674 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
ZYGOSPIRA sp. 1 
Plate 186, B, figures 9, 10 
Shell of about medium size for the genus, longer than wide, with an elongate 
oval outline ; surface costellate, marked by about 24 narrowly rounded costellae 
separated by spaces about equal in width to the width of the costellae. 
Pedicle valve strongly convex in lateral profile and with the maximum con- 
vexity near the middle; anterior profile narrowly rounded and with long con- 
cave lateral slopes. Umbonal and median regions narrowly swollen. Fold con- 
spicuous but not clearly demarcated from the flanks. Fold marked in center by 
a stria wider than the others. 
Brachial valve gently convex in lateral profile; anterior profile sulcate; sul- 
cus wide and deep, occupied by 9 costellae, the median one of which is larger 
than the others and corresponds to the wide striation on the fold of the pedicle 
valve. Flanks narrowly rounded and inflated. 
Measurements in mm.— 
Brachial 
Length length Width Thickness 
Hypotype (pedicle valve 117262a)... 4.2 rd aur 1.0 ? 
- (brachial valve 117262b).. ? 3.4 3.3 0.6 ? 
Figured specimens.—117262a,b. 
Horizon and locality—Gray granular limestone lenses in the Sevier formation 
in Tennessee: On the hill slope on the west side of the church in Guthrie Gap, 
Bulls Gap (T.V.A. 171-SE) Quadrangle. 
Genus PROTOZYGA Hall and Clarke, 1893 
Protozyga Haut and CLARKE, Pal. New York, vol. 8, pt. 2, pp. 149-151, 1893. 
Usually small to minute, subcircular to oval in outline, biconvex, with the 
pedicle valve generally the deeper. Pedicle valve with more or less pronounced 
fold usually flattened or depressed longitudinally by a narrow sulcus. Brachial 
valve sulcate and with the sulcus usually occupied by a more or less prominent 
fold corresponding to the median depression of the fold on the pedicle valve. 
Flanks smooth or paucicostate. Beak usually incurved, foramen elongate-oval 
surrounded anteriorly and laterally by deltidial plates. Shell structure impunctate. 
Pedicle interior with small teeth buttressed by well-developed dental plates 
forming a deep delthyrial cavity. Muscle field heart-shaped. 
Brachial interior with zygospiroid hinge plate consisting of narrow, triangu- 
lar socket plates to which the crura are attached. A low median ridge divides 
the adductor scars but does not extend posteriorly to the hinge plate. Crura 
very short, terminating in crural processes ; descending lamellae long, diverging 
laterally but converging toward their anterior ends. Spire variable, consisting of 
part of a volution to nearly a whole volution. The descending branches are united 
by a stout jugum directed more or less posterodorsally. 
Genotype.—Atrypa exigua Hall, Pal. New York, vol. 1, p. 141, pl. 33, fig. 6, 
1847. 
