PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 297 
Shippensburg formation in Pennsylvania: Railroad cut 1.7 miles southwest 
of Marion, Chambersburg (15’) Quadrangle. 
Shippensburg formation (Pinesburg member—shaly beds between Echino- 
sphaerites and lower Nidulites zones) in Maryland: On U. S. Highway 40, just 
west of Conococheague Creek, 7 miles west of Hagerstown, Williamsport (15’) 
Quadrangle. 
Discussion —Orthambonites bielsteini most nearly resembles O. buttsi Schuch- 
ert and Cooper and O. mostellerensis, new species. From the former it differs 
in its less transverse outline and less prominent median sulcus on the brachial 
valve. From the latter O. bielsteini differs in having less numerous and more 
narrowly elevated costae. 
ORTHAMBONITES BIFURCATUS Cooper, new species 
Plate 34, A, figures 1-6 
Shell of about medium size for the genus, wider than long with the hinge con- 
siderably narrower than the greatest shell width which is at the middle. Cardi- 
nal extremities obtuse. Sides fairly strongly rounded; anterior margin broadly 
rounded. Surface marked by narrow, elevated, subangular costae that bifurcate 
at various distances from the beak. Including bifurcations, about 60 costae occur 
along the anterior margin. 
Pedicle valve with lateral profile moderately convex and with the greatest 
curvature in the posterior half. Anterior profile moderately convex with long 
and moderately steep slopes. Posterior margins curving away from the beak 
which protrudes conspicuously. Umbo swollen and continued anteriorly as a 
low fold to the front margin. Lateral areas sloping gently to the margins, the 
slopes steepening to the umbonal slopes which are short and abrupt. Interarea 
short, curved, apsacline. 
Brachial valve less deep than the pedicle valve, gently convex in both profiles. 
Median sulcus originating at the umbo and extending to the anterior margin, 
deepening and widening anteriorly to occupy about one-third the width at the 
anterior margin. Flanks gently swollen with short concave and steep lateral 
slopes. Posterolateral extremities flattened. Interarea short. 
Measurements in mm.— 
Hinge 
Length Width width Thickness 
PPOIDE. cyst celoyeis evans» ois ¥ arden < 11.7 14.6 10.4 5.7 
Paratype (100798g) ........... 10.0 12.3 9.2 5.0 
$ (100 708d) i aes 9.6 12.3 8.9 4.4 
Types——Holotype: 109798e; figured paratype: 109798g, 116869; unfigured 
paratypes: 109798a-d,f,h,i. 
Horizon and locality—Upper Pogonip group (Rhysostrophia zone) in Ne- 
vada: North side of Ikes Canyon about 1 mile above the entrance, east side of 
Toquima Range, Roberts Mountains (1°) Quadrangle. 
Discussion—This species is characterized by the strong bifurcation of the 
costae, a feature that will distinguish it from most members of the genus. It differs 
