PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 785 
poorly differentiated costellae, and the presence of a strong plica or fold on the 
pedicle valve. This species is smaller than S. silicica which is not so strongly 
folded. It is suggestive of S. varicostellata but is also smaller than that species, 
more strongly concave, more strongly folded in the median part of the pedicle 
valve, and has stronger costellae. 
SOWERBYELLA MERRIAMI Cooper, new species 
Plate 269, D, figures 18-22 
Shell of about medium size for the genus, wider than long and with the hinge 
forming the greatest width; sides slightly rounded, sloping medially ; anterior 
margin strongly rounded; anterior commissure rectimarginate. Surface costel- 
late, costellae of unequal size, 7 to 9 of the strongest ones in 5 mm. at the front 
margin, each of the stronger costellae separating I to 5 of the finer ones. Pos- 
terior margins with numerous irregular, oblique wrinkles. 
Pedicle valve gently convex in lateral profile, with the greatest convexity in 
the posterior third and with the anterior two-thirds flattened. Anterior profile 
subcarinate. Umbo narrowly swollen; umbonal slopes short and gentle, melting 
into the flattened flanks and medial portion. Interarea strongly apsacline and 
moderately long. Pseudodeltidium small. 
Brachial valve nearly flat except in the umbonal region which is deeply con- 
cave, the concavity extending along the posterior margin and dying out on the 
flanks and median region, lateral and anterior regions nearly flat. 
Interior unknown. 
Measurements in mm.—Holotype, length 11.6, length of brachial valve 11.0, 
midwidth 17.3, hinge width 18.9, thickness 2.1. 
Types.—Holotype: 124234a; unfigured paratypes: 124234b-d. 
Horizon and locahity—Dark shale under the Eureka quartzite in Nevada: 
Under two knobs of Eureka quartzite, north side of canyon, 3.1 miles N. 32° E. 
of the Blair (Segura) Ranch, Antelope Mountains, Roberts Mountains (1°) 
Quadrangle. 
Discussion.—This species can be recognized by its fairly large size, wide hinge, 
strong ornamentation, and flattened form. It is suggestive of S. eximia from 
contemporaneous rocks in Virginia but is more strongly ornamented and flatter 
in profile. Sowerbyella monilifera resembles the Nevada species but is usually 
smaller, less extended along the hinge, and has finer ornamentation. 
SOWERBYELLA MONILIFERA Cooper, new species 
Plate 199, A, figures 1-8; plate 206, E, figures 16-28 
Shell large, subrectangular to subelliptical in outline, wider than long, with 
the length about two-thirds the width; cardinal extremities angular to auriculate. 
Sides gently rounded, moderately oblique; anterior margin broadly rounded; 
costellae of several sizes but with a prominent strong set in a field of fine ones; 
8 of the stronger costellae occupy 5 mm. at the front margin; larger costellae 
separated by 1 to 6 of the fine costellae, which are beaded. 
