PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 673 
commissure strongly sulcate. Surface marked by about 18 or 19 strong costae, 
5 in the sulcus and 6 on the fold. 
Pedicle valve strongly convex in lateral profile; greatest convexity at or 
slightly anterior to the middle; anterior profile narrowly convex and with gently 
convex, steeply sloping sides. Umbo narrowly swollen; median region narrowly 
swollen to form a prominent fold which is not clearly demarcated from the flanks. 
Fold marked by a median striation wider and deeper than the others; flanks 
gently swollen. Beak long and incurved. 
Brachial valve gently convex in lateral profile but with the posterior half 
somewhat flattened and the anterior half moderately convex. Anterior profile 
broadly sulcate; sulcus broad and shallow, its middle marked by a costa stronger, 
wider, and more elevated than the surrounding ones, which corresponds to the 
median striation of the fold of the pedicle valve. Flanks narrowly rounded. 
Measurements in mm.—Holotype, length 5.0, brachial length 4.5, width 4.3, 
thickness 2.6. 
Types.—Holotype: 117260a; unfigured paratype: 117260b. 
Horizon and locality.—Sevier formation in Tennessee: In wagon road just 
northwest of the divide, 14 miles S. 60° W. of Bulls Gap, Mohawk (T.V.A. 
172-NE) Quadrangle. 
Discussion.—This species is most suggestive of Z. lebanonensis but important 
differences exist. The sulcus of Z. mediocostellata is longer, deeper and wider 
than that of the Lebanon species, and the widest part is posterior to the middle 
rather than anterior to the middle as in Z. lebanonensis, The flanks of the latter 
shell are more swollen, and the median costella and corresponding striation are 
less prominent. 
ZYGOSPIRA RECURVIROSTRIS (Hall) 
Plate 142, H, figures 34-38 
Figures of this widely misidentified species are given for comparison with the 
Appalachian and other species which are mostly older stratigraphically. 
Figured specimen.—1 11380. 
Horizon and locality.—Trenton formation in New York: At Martinsburg, 
Port Leyden (15’) Quadrangle. 
ZYGOSPIRA VARIABILIS Fenton and Fenton 
Plate 142, E, figures 17-25 
Zygospira variabilis FENTON and Fenton, Proc. Iowa Acad. Sci. for 1922, vol. 20, p. 75, 
pl. 2, figs. 7-9, 1924. 
Types.—Figured hypotypes: 111387a-c,e. 
Horizon and locality.—Plattin group (Macy formation-Zell member) in Mis- 
souri: South Beckett Hill, Weingarten (15’) Quadrangle. 
Discussion.—A silicified specimen in the National Museum collections shows 
the descending lamellae of the spire and the transverse ribbon in the same posi- 
tion as Hall and Clarke describe it for Z. modesta and some forms of Z. recurvi- 
rostra. 
