PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 347 
face costate, costae numbering about 34 in an adult. Costae not greatly elevated, 
somewhat narrowly rounded and separated by spaces equal in width to the 
costae. Interspaces finely costellate. 
Pedicle valve gently convex in lateral profile with the greatest convexity lo- 
cated in the posterior half; anterior half somewhat flattened. Anterior profile 
broadly convex. Median region swollen to form an indistinct fold. Lateral slopes 
slightly convex, not steep. Cardinal extremities somewhat flattened. Interarea 
moderately long, strongly apsacline. 
Brachial valve moderately convex in lateral profile; anterior profile broadly 
convex with short lateral slopes and a median depression. Median sulcus origi- 
nating at the umbo, narrow, shallow, and extending for about three-quarters the 
length where the valve is swollen slightly. Flanks broad and somewhat swollen 
with short, gentle lateral slopes. Cardinal extremities somewhat flattened. Inter- 
area short, anacline. 
Measurements in mm.—Holotype, length 17.1, brachial length 16.3, width 21.5, 
hinge width 20.0, thickness 8.6. 
Types.—Holotype: 109962; figured paratype: 109964. 
Horizon and locality —Lincolnshire formation in Tennessee: 4 mile northwest 
of Eidson, Kyles Ford (T.V.A. 170-SE) Quadrangle. 
Discussion—This is a large species than can be recognized by its strongly 
apsacline pedicle palintrope, a brachial valve that is unusually convex for the 
genus, and by the large number of costae. Its nearest related form is H. longi- 
rostris, but this differs in the greater length of the pedicle palintrope, the con- 
cave to flat brachial valve, less numerous costae, and different outline in the adult. 
HESPERORTHIS COLEI Cooper, new species 
Plate 54, D, figures 16-21 
Shell of about medium size for the genus, length and width about equal ; cardi- 
nal extremities slightly obtuse or about a right angle; sides nearly straight ; an- 
terolateral areas gently rounded; anterior margin fairly strongly rounded; 
anterior commissure slightly sulcate; surface marked by 32 to 33 somewhat 
broadly rounded costae separated by spaces narrower than the costae. Strong 
concentric fila and fine radial lines occupy the striae. 
Pedicle valve moderately convex with the greatest convexity in the posterior 
third; anterior two-thirds moderately steep and gently convex; anterior profile 
somewhat narrowly convex and with flat, steep slopes to the margins. Umbo 
narrowly swollen, median region gently inflated; anterior slope long; postero- 
lateral slopes steep. Interarea long, strongly curved, strongly apsacline. Interior 
with large teeth, deep fossettes ; dental plates short and receding ; delthyrial cavity 
deep ; muscle region somewhat elongated, somewhat thickened. Pseudodeltidium 
exceptionally long for the genus; lateral plates well developed, marked by an 
elevated ridge on each side of the delthyrium. 
Brachial valve unevenly concave in lateral profile, the greatest concavity lo- 
cated in the posterior third, the anterior two-thirds flat to gently concave ; umbo 
concave; sulcus shallow, originating at the umbo, widening anteriorly and form- 
