PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 459 
Types.—Figured hypotypes: 24772a, 24760; unfigured hypotypes: 24772b, 
110588a. 
Horizon and locality—Decorah formation (Spechts Ferry member) in Min- 
nesota: At Fountain, Fillmore County; Chatfield, Fillmore County; St. Paul; 
Minneapolis ; Elmira, Olmsted County. 
Rockland formation in Ontario, Canada: At Gloucester and near Ottawa. 
Rockland formation in New York: At Watertown and 1 mile east of Dexter. 
Macy formation (Zell member) in Missouri: On the old road 1 mile north- 
west of Chicago Summer School Camp, NW1SW43 sec. 32, T. 27 N., R.9 E., 
Weingarten (15’) Quadrangle. 
Discussion.—This species is distinguished by its fairly large size and the strong 
development of the fold and sulcus particularly in the anterior third or half. As 
here identified the species is very variable as to outline and profile, development 
of fold, and sulcus and ornamentation. As would be expected, variations in pro- 
file are common and may have nothing to do with the biological characters of the 
shells. Some of the specimens occur in soft shales and may be somewhat flattened. 
Variations in outline are not generally due to distortion, and specimens in all 
types of matrix showed a variation in the relation of length to width. Some 
specimens are distinctly more rounded than others, and in the round ones the 
cardinal extremities are generally much rounded, a feature that emphasizes the 
rotundity of the outline. Some specimens are distinctly shouldered and have 
cardinal extremities approaching a right angle. Most of the specimens referred 
to this species have a fairly narrow fold and sulcus, but in a few of them, the 
sulcus may occupy as much as half the shell width. Specimens of this type occur 
in Minnesota and Ontario, but none was seen in the Plattin group. The Minne- 
sota specimens vary from the Canadian ones in the character of the ornamenta- 
tion, particularly the costellae occupying the sulcus. In the Minnesota forms 
these costellae are generally nearly uniform in size, but in the Canadian speci- 
mens a tendency to alternation of fine and strong costellae is present. Despite 
these variations the writer was unable to find suitable differences on which to 
base additional species. 
DOLEROIDES IRREGULARIS Cooper, new species 
Plate 95, E, figures 31-39 
Shell small for the genus, wider than long, with the greatest width located an- 
terior to the middle; hinge narrow; sides oblique outward; anterolateral extremi- 
ties narrowly rounded; anterior margin nearly straight; anterior commissure 
slightly uniplicate ; costellae unequal in size, those of the flanks finer than those 
in the middle, about 12 in 5 mm. at the margin. 
Pedicle valve somewhat less deep than the brachial valve and with unevenly 
convex lateral profile, the posterior half fairly strongly convex, anterior half 
flattened. Anterior profile broadly but fairly gently convex; beak prominent ; 
umbo somewhat narrowly rounded; median region moderately swollen ; anterior 
half to third depressed into a broad, shallow sulcus; flanks bounding sulcus 
