PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 757 
between these and the fine costellae covering most of the surface are intercalated 
and become most numerous on the anterior and anterolateral slopes. Eleven or 
twelve of these costellae occupy a space of 5 mm. at the front margin. As many 
as 4 of the finest costellae occupy the space between the intermediate costellae. 
Pedicle valve moderately convex in lateral profile with the greatest convexity 
at about the middle in young specimens but in the posterior half in old speci- 
mens. Front profile variable depending on the age; most strongly convex in the 
older individuals. Beak inconspicuous; umbo small, slightly swollen, extended 
forward as an ill-defined fold on the flattish posterior portion, but generally not 
continued far forward, although some specimens show evidence of an indistinct 
fold at the front margin. Posterolateral slopes gently concave. Anterolateral 
slopes generally steep in shells of all ages; front slope steep to moderately steep 
depending on age, the older shells having a longer and more gentle anterior slope. 
Interarea only slightly curved, apsacline to orthocline. Pseudodeltidium mod- 
erately long, narrowly arched. 
Brachial valve following the curvature of the pedicle valve fairly closely, most 
deeply concave at the middle or slightly posterior to the middle. Umbo gently 
concave with the concavity produced anteriorly to about the middle. Postero- 
lateral regions flattened. Cardinal extremities sulcate. Interarea short, hy- 
percline. 
Interior of pedicle valve with small teeth, dental plates strongly convergent, 
thick ; umbonal cavities well filled by callus. Delthyrial cavity moderately deep ; 
muscle marks well impressed on a thickened delthyrial floor. Diductor-adjus- 
tor scars small, often separated by a low ridge at the posterior of which is located 
the small oval adductor field. Vascula media often much thickened, short, having 
the first bifurcation at about the middle of the valve. 
Brachial valve with a visceral disk occupying about two-thirds the width and 
more than half the length. Median ridge strongly elevated at the front, lost in 
the callus under the cardinalia. Cardinal process and chilidial plates forming a 
flat piece supported anteriorly by a thickening of callus. Brachial processes long, 
flat plates. Adductor scars not discernible. 
Measurements in mm.— 
Brachial Surface Mid- Hinge Thick- 
Length length length width width ness Height 
BAI Gt Ce bess. she steteberots 10.5 8.2 13.5 10.4 12.0 3.2 4.4 
Hypotype (108135s).. 10.4 8.3 12.5 12.3 TOG. 1316 5.6 
se (108135h).. 11.4 9.0 16.0 12.5 re q2 5.8 
as (108139)... 11.8 10.3 15.5 14.8 14.6 2.6 5.4 
Types.—Holotype: 92871 ; figured hypotypes: 108135d,e,g,h,m,o,p,s-u, 117452, 
117453a,b, 117454a,b, 117455a,b, 117456a, measured hypotype: 108139. 
Horizon and locality.—Arline formation in Tennessee: On both sides of an 
old road in glade, 4 mile southeast of Friendsville, Concord (T.V.A. 138-SW) 
Quadrangle ; 3 and 6 miles southeast of Knoxville; McMullens, Meadow (T.V.A. 
139-NW) Quadrangle; 1,500 feet south of the mouth of Burnett Creek, Shooks 
Gap (T.V.A. 147-NE) Quadrangle; 6 to 10 miles southeast of Loudon, Loudon 
