670 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
vician. In the Central Basin of Tennessee it is abundant in the Lebanon and 
Carters formations. It has not yet been seen in the Simpson group. 
From the foregoing it is evident that Zygospira has a Wilderness or Trenton 
age. Rocks in Pennsylvania assigned to the “Chazyan” or “Stones River” and 
containing Zygospira are almost certainly incorrectly dated. 
ZYGOSPIRA CIRCULARIS Cooper, new species 
Plate 141, C, figures 18-21; plate 142, B, figures 6-10; plate 142, D, figure 16 
Shell of about medium size for the genus, nearly circular in outline, without 
pronounced shoulders ; beak obtuse, forming an angle of 110° ; sides and anterior 
margin rounded. Anterior commissure gently and broadly sulcate. Fold and 
sulcus not well developed. Costae numbering 22, narrowly rounded and having 
a width about equal to that of the interspaces. 
Pedicle valve strongly convex in lateral profile with the convexity slightly 
greater in the posterior half. Anterior profile narrowly convex. Umbo narrowly 
convex; median region somewhat narrowly swollen to form an indistinct fold ; 
flanks slightly concave and with steep slopes to the margins. 
Brachial valve with about half the depth of the pedicle valve, moderately and 
evenly convex in lateral profile, broadly and gently convex in anterior profile; 
umbonal region slightly convex ; sulcus indistinct ; flanks somewhat swollen and 
with moderately steep slopes to the cardinal extremities. Median costa of the 
brachial valve enlarged and sunk slightly below the level of those surrounding it. 
Measurements in mm.—Holotype, length 5.9, brachial length 5.3, width 5.6, 
thickness 3.7. 
Types.—Holotype: 111374a; figured paratypes: 111385, 117259; unfigured 
paratype: 111374. 
Horizon and locality—Carters formation in Tennessee: Near Franklin, 
Franklin (15’) Quadrangle; on Tennessee Highway 58, 1 mile south of Wood- 
bury, Woodbury (15’) Quadrangle; east side of U. S. Highway 70S, 4.2 miles 
southeast of Nashville; north side U. S. Highway 70S, east of Hill Branch, 
1 mile east of Woodbury, Woodbury (15’) Quadrangle; north side of U. S. 
Highway 70S, 0.8 mile east of Readyville, Cannon County. 
Top of Lebanon formation: In quarry on U. S. Highway 41, 11.7 miles south- 
east of Murfreesboro, Rutherford County. 
Discussion.—In the possession of an enlarged median rib and corresponding 
median depression on the pedicle valve this species resembles Z. variabilis Fen- 
ton and Fenton but differs in its more rounded outline and indistinctness of the 
fold and sulcus. 
ZYGOSPIRA ELONGATA Cooper, new species 
Plate 268, G, figures 29-32 
Shell of about medium size for the genus, longer than wide, with a subpen- 
tagonal outline. Beak obtuse; posterior margins short, connecting the beak with 
narrowly rounded shoulders and gently rounded lateral margins. Anterior mar- 
