PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 349 
Brachial valve fairly strongly concave in lateral profile with the deepest part 
slightly posterior to the middle; umbo gently concave, narrowly sulcate; sulcus 
narrow, expanding gradually anteriorly but never attaining much width; sulcus 
extending directly to the anterior margin and containing 2 costae; areas bound- 
ing sulcus nearly flat; posterolateral areas gently concave. Interior with long 
brachiophores and long, low, median ridge. 
Measurements in mm.—Holotype, length 14.8, brachial length 12.5, width 
16.2, hinge width 14.8, thickness, 7.0. 
Types.—Holotype: 83842; figured paratype: 116917. 
Horizon and locality.—Platteville formation (McGregor member) in Wiscon- 
sin: From a quarry on the south side of Ellenboro Hill halfway between Lan- 
caster and Platteville; quarry on Wisconsin Highway 81, 1 mile northwest of 
Ellenboro; on road 3 miles northwest of Platteville; Lancaster (30’) Quad- 
rangle. 
Platteville formation in Illinois: 14 miles northeast of Dixon. 
Chaumont formation in New York: Loose in the bed of Black River, just be- 
low the dam at the head of the island, Watertown, Watertown (15’) Quadrangle. 
Discussion—This species is characterized by its strongly inflated pedicle valve 
which is not subcarinate as is usual in many species of Hesperorthis, by the fairly 
strongly concave brachial valve with its narrow sulcus, and by the peculiar nar- 
row fold produced by the slightly elevated median rib. These features distinguish 
it from H. australis and from H. colei, which is more finely costate and not so 
strongly convex. 
HESPERORTHIS ? COSTALIS (Hall) 
Plate 32, E, figures 9-15 
Orthis costalis Hatt, Pal. New York, vol. 1, p. 20, pl. 4, fig. 4a, 1847—RayMonp, Ann. 
Carnegie Mus., vol. 7, p. 235, pl. 35, fig. 4, 1911.—BassLer, U. S. Nat. Mus. Bull. 92, 
p. 888, 1915. 
Raymond fixed the type of this species on an imperfect specimen occurring on 
a piece of dark limestone containing a fragment of a Strophomena. Raymond 
claims that the specimen which is from Chazy, N. Y., could have come only from 
Middle Chazy or Black River rocks, the only two formations having lithological 
characters like the fragment containing these specimens. Raymond also concludes 
that the specimen is closer to Hesperorthis tricenaria than to any other species 
then described from the Chazyan. It is certain that the specimen is not like any 
named Chazy form and that it has some of the characters of Hesperorthis such 
as the musculature and the interarea. For the present the name will have to rest 
on that specimen, and definition of its horizon and biological characters will have 
to await discovery of additional material. 
Types.—Holotype: A.M.N.H. 528; figured hypotypes: 116853b-e,j; un- 
figured hypotypes: 116853a,f-i,k-m. 
Horizon and locality—Day Point formation (upper “orthis” bed) in New 
York: Quarry just north of Hotel Champlain, south of Cliff Haven, Plattsburg 
(15’) Quadrangle. 
