PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 359 
Horizon and locality—Decorah formation (Guttenberg member) in Wiscon- 
sin: Mineral Point; 0.1 mile south of the Little Platte River on Wisconsin 
Highway 81, 2 miles northwest of Platteville, Mineral Point (30’) Quadrangle; 
3 miles northwest of Platteville, Lancaster (30’) Quadrangle. 
Same formation in Illinois: At Rocton. 
Lower Trenton limestone in New York: At Middleville. 
Barnhart formation in Missouri: On the old road 1 mile northwest of Chicago 
Summer Camp, NW4SW3 sec. 32, T. 37 N., R. 9 E., Weingarten (15’) Quad- 
rangle ; at Koch Valley School on U. S. Highway 61, NE. cor. SW4SW3 sec. 
6, T. 41 N., R. 7 E., 2 miles south of Barnhart, Kimmswick (15’) Quadrangle; 
on U. S. Highway 61, 0.1 mile south of Salt River, north of New London, Ralls 
County. 
Auburn chert in Missouri: $ mile east of Auburn, Elsberry (15’) Quadrangle. 
Rockland formation in Ottawa, Canada: At Paquette Rapids. 
Curdsville formation in Kentucky and Tennessee. 
Discussion.—This species is characterized by its large size, being one of the 
largest members of the genus to occur in the Middle Ordovician. It is character- 
ized, besides its size, by its nearly parallel sides, shallow, narrow sulcus, and 
indistinct pedicle fold. Although the type specimen is lost, it is possible to 
identify the species in the Middle Ordovician sequence of southwestern Wiscon- 
sin by its reported large size. The horizon of this large form is the lower part 
of the Guttenberg above the thin band of blue clay so conspicuous in this part of 
Wisconsin. The species is also conspecific with the abundant large Hesperorthis 
of the Curdsville in Kentucky and its equivalents in southwestern Virginia and 
eastern Tennessee. The species is also abundant in the upper part of the Plattin 
limestone group and the so-called Decorah of eastern Missouri. 
Hesperorthis disparilis (Conrad) is a small species that has for some years 
been placed in the synonymy of H. tricenaria. This seems to be the correct place 
for the species because the type specimen appears to be a young form. 
HESPERORTHIS VIRGINIENSIS Cooper 
Plate 51, D, figures 19, 20; plate 82, G, figures 28-32 
Shell of about medium size for the genus, length and width about equal ; cardi- 
nal extremities approximately a right angle, hinge slightly narrower than the 
greatest shell width at about the middle; anterior commissure gently sulcate; 
sides nearly straight to slightly rounded; anterior margin strongly rounded ; sur- 
face marked by 30 rounded costae. 
Pedicle valve gently convex in lateral profile with the greatest convexity in 
the posterior half ; anterior profile narrowly convex and with long, steep slopes ; 
umbo narrowly swollen, the swelling occupying the median region from the 
beak to the margin, forming a poorly defined fold ; lateral slopes steep. Interarea 
strongly apsacline to nearly catacline. 
Brachial valve almost plane in lateral profile; gently concave in anterior pro- 
file; umbonal region sulcate, the sulcus deepening and widening anteriorly to 
