PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 735 
anterior margin, deeply impressed and narrow diductor impressions, and wide, 
shallow adductor field. Delthyrial cavity filled with callus; anterior portion of 
muscle area thickened and elevated. Visceral region much thickened and with 
many oblique subperipheral ridges. 
Interior of brachial valve with thickened median septum rising to a sharp 
crest near the middle of the valve and composed of 2 nearly parallel septa 
cemented together by shell material. Adductor impressions short and narrow, 
deeply impressed. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
ELOLOUV DEY GRO ee aes NE 10.5 9.4 Tst7. 14.7 3.4 
Paratype, «GLLOOOS))) cyt. sty aed « Gy2 0.6 ? 12.2 13.0 1.8 
as GETOOOSE ))i5.4:< nps,0'e sexes 8.4 7.4 11.8 14,1 3.0 
Hi REPOOOSE)) ogee s.s 5 aides 2 7.4 10.7 13.0 ? 
Types.—Holotype: 110905a; figured paratypes: 110go5b-d, 1173738, 
117374a,b; unfigured paratypes: 110904, 117373b. 
Horizon and locality.—Benbolt formation in Virginia: At 4o feet above the 
base, + mile southeast of Green Valley Church, 3 miles southwest of Lebanon, 
Brumley (T.V.A. 205-SE) Quadrangle; 4 mile northeast of Speers Ferry, 
Clinchport (T.V.A. 188-NW) Quadrangle; 0.2 mile south of the middle fork 
of Moccasin Creek, 14 miles south of Hansonville, Hansonville (T.V.A. 205-SW) 
Quadrangle. 
Botetourt formation in Virginia: On the south side of U. S. Highway 60, 
1 mile northwest of Lexington, Lexington (15’) Quadrangle; at the junction of 
Virginia Highways 731 and 724, 2 miles northwest of Brownsburg, Lexington 
(15’) Quadrangle. 
Arline formation in Virginia: Quarry at Marion, Marion (T.V.A. 218-SE) 
Quadrangle. 
Discussion.—This species differs from other known Appalachian species by 
its relatively greater length, length/width ratio varying from 0.70 to 0.85. The 
brachial interior is somewhat less thickened and the callus swelling about the 
median septa much less developed than in J. ricevillense which most nearly ap- 
proaches this species in size. This Benbolt species will not be confused with the 
Arline I. extensum which is a quite slender form usually of small size. The 
I. subabbreviatum again is smaller and possesses a median structure much more 
highly elevated in the direction of the pedicle valve than that of J. biseptatwm. 
ISOPHRAGMA EXTENSUM Cooper, new species 
Plate 170, C, figure 17; plate 170, D, figures 18-29; plate 171, A, figures I-14; 
plate 171, C, figures 16-19; plate 184, F, figures 19-22 
Shell of about medium size for the genus, nearly twice as wide as long at the 
hinge and with the hinge forming the widest part. Cardinal extremitites acute 
to alate. Front margin broadly rounded to truncate. Surface marked by costellae 
of unequal size; 2 to 4 of the smaller costellae occupy the space between the 
