686 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
PROTOZYGA SUPERBA Cooper, new species 
Plate 141, L, figures 63-69 
Zygospira nicolleti Winchell and Schuchert, Forrste, Bull. Sci. Lab., Denison Univ., vol. 10, 
No. 3, p. 199, pl. 23, figs. 8A, 8B, 1920. 
Adult fairly large for the genus, pentagonal to subcircular in outline; beak 
acute to obtuse, strongly incurved; lateral margins rounded; anterior margin 
narrowly rounded to subtruncate. Surface smooth except for 2 or 3 costae de- 
veloped along the front margin on each side of the fold and sulcus. 
Pedicle valve with nearly twice the depth of the brachial valve, strongly con- 
vex in lateral profile with the greatest convexity in the posterior half in the adult ; 
umbo narrowly swollen ; fold originating on the umbo and extending as a rounded 
plica to the middle of the valve where a shallow depression develops in the center 
of the fold and extends to the front margin; flanks nearly flat in profile and with 
steep slopes to the margins. 
Brachial valve evenly and moderately convex in lateral profile with the great- 
est convexity at about the middle; umbonal, median region, and flanks some- 
what swollen; sulcus originating on the umbo, where it is poorly defined and 
shallow, becoming deeper and wider near the middle and occupying nearly half 
the width at the front margin. One or two costae appearing anteriorly in the 
sulcus which correspond to the sulcus in the fold of the pedicle valve ; sulcus 
drawn anteriorly and toward the pedicle valve into a moderately long, squarely 
truncated tongue. Flanks convex in profile and slopes to the margins moderately 
steep. 
Interior: Dental plates of pedicle valve well developed ; adductor muscle tracks 
of brachial valve long and slender. 
Measurements in mm.— 
Length Width Thickness 
FIOISEV DEN, 0:24 Mise Heated eR apn toad tippers 5.3 4.9 3-4 
Pagatyne (CRI VARS D sn acy oxucie ae Mole Ot eat. 4.1 4.2 2.8 
y CEREAS SC Me ote © ase eehebal ale ale 0 ok 4.6 4.5 2.2 
Types.—Holotype: 111455a; unfigured paratypes: 111455b,c. 
Horizon and locality—Auburn chert in Missouri: 4 mile east of Auburn, 
Elsberry (15’) Quadrangle. 
Top of Plattin group (=Barnhart formation) in Missouri: Buford Cave, 
Ralls County; Yeager Farm, south edge of Ralls County; Conns Ford, Ralls 
County. 
Discussion——This species resembles P. nicolleti but differs in possessing a 
more convex pedicle valve, fewer costae on the flanks, a deeper sulcus, and a 
lesser development of the fold within the sulcus of the brachial valve. This latter 
feature is the most striking difference. In the Minnesota species the fold may 
originate in the sulcus posterior to the middle of the valve, but in P. superba the 
fold within the sulcus occurs in the front part of the sulcus and on the tongue. 
