PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 353 
margins. Interarea long, steeply apsacline, slightly curved. Pseudodeltidium 
present but a mere vestige. Interior with short, receding dental plates; teeth 
small; lateral plates narrow; vascula media convergent medially, not strongly 
developed. 
Brachial valve slightly convex in lateral profile but perceptibly convex in an- 
terior profile. Sulcus originating at the umbo, widening rapidly anteriorly but 
only slightly perceptible at the anterior margin. Flanks bounding the sulcus 
slightly convex; cardinal extremities gently deflected toward the pedicle valve. 
Interarea fairly long, anacline. Interior with stout median ridge, moderately 
thickened notothyrial platform, and moderately long brachiophores. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Piolo ype He Hoya Pet ts CIO 10.7 14.4 13.8 7.5 
Paratype (98180e) .......... 10.2 0.4 13.2 13.0 6.0 
RIGO0SAd)) Eons nee 14.4 12.0? 16.7 15.1 6.5 
Types.—Holotype: 98180a; figured paratypes: 116920e, 1099341; unfigured 
paratypes: g8180b-e, 116920a-d,f, 109934a-h,j-t. 
Horizon and locality——Whistle Creek formation in Virginia: 14 miles north- 
west of Lexington on Whistle Creek and on U. S. Highway 60, 100 yards south- 
east of Whistle Creek, Lexington (15’) Quadrangle; Virginia Highway 602, 54 
miles S. 82° W. of Middlebrook, Augusta County. 
Ellett formation in Virginia: Ellett and Lusters Gate, Blacksburg (15’) Quad- 
rangle. 
Lincolnshire formation in Tennessee: 4 mile northwest of Eidson, Kyles Ford 
(T.V.A. 170-SE) Quadrangle; Evans Ferry. In Virginia: At Marcem Quarry, 
2 miles west of Gate City, Gate City (T.V.A. 188-NE) Quadrangle. 
Discussion.—This species resembles H. australis but generally in its adult state 
is somewhat larger. The brachial valve is generally flat or convex, seldom con- 
cave in the adult. The most distinguishing feature is the interarea of H. longi- 
rostris which is much longer than that of H. australis. The greater length of 
the interarea makes H. longirostris considerably thicker in the posterior half 
than is the stratigraphically higher species. 
Although the outline and profile of H. longirostris approach those of H. sulcata, 
the two species may be distinguished by the more prominent sulcus of the latter 
and the longer interarea of H. longirostris. 
HESPERORTHIS MATUTINA Cooper, new species 
Plate 54, A, figures 1-4 
Shell of about medium size for the genus, wider than long, with nearly straight 
lateral margins and cardinal extremities approximating a right angle. Front 
margin broadly rounded. Surface marked by 25 to 27 elevated, rounded costae 
separated by interspaces about half the width of the costae. Fine radial costellae 
appearing indistinctly between the costae. 
Pedicle lateral profile with greatest convexity in the posterior half; anterior 
