534 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Horizon and locality—Mingan formation (about 25 feet above the base ac- 
cording to Twenhofel and Whiting) on Quarry and Quin Islands, Mingan group, 
St. Lawrence River, Quebec, Canada. 
Discussion.—This species is characterized by its short fold, rotund and tumid 
brachial valve, and moderately acute pedicle beak. It is not a common species, 
according to reports, and is confined to the lower part of the Mingan formation 
on two islands of the Mingan group. The distinctions between this species and 
O. gracilis Raymond are stated under that species. This is the only described 
species with which the Mingan form might be confused. 
ONYCHOPLECIA MATUTINA Cooper, new species 
Plate 121, F, figures 15-17 
Shell small, suboval in outline, apical angle about 100°; sides rounded; an- 
terior commissure uniplicate; surface smooth but where exfoliated, shell is 
marked by radial lines as usual in the Triplesiidae. 
Pedicle valve moderately convex in lateral profile; somewhat narrowly con- 
vex in anterior profile; umbo somewhat narrowly swollen; median region in- 
flated; sulcus originating at about the middle, short and deep and occupying 
about half the width. Flanks bounding sulcus narrowly swollen and with steep 
slopes. 
Brachial valve moderately convex in anterior and lateral profile; umbonal 
region swollen; fold originating just posterior to the middle, narrowly convex in 
the middle and with moderately sloping sides. Flanks moderately inflated. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
Ficlotype* Fa s.cd ote secs oa eit 57) ? 6.8 ? 12:5 
Paratype \(1rT7oarb)ihas. ace cies ? 5.0 6.3 ? 0.8 ? 
Types.—Holotype: 117940a; figured paratypes: 117941a,b. 
Horizon and locality—Basal Everton formation in Arkansas: At the north- 
west edge of Pindall, near the school, Yellville (30’) Quadrangle. 
Jasper formation in Arkansas: Just northwest of Pindall, Yellville (30’) 
Quadrangle. 
Discussion.—This species is unlike any others of this genus described in its 
transversely elliptical form, short, wide-angled beak, short deep sulcus and mod- 
erately convex brachial valve. The species is of note because it is one of the few 
fossils found in the Everton and Jasper formations. 
ONYCHOPLECIA OBESA Cooper, new species 
Plate 100, E, figures 23-27 
Large for the genus, broadly oval in outline; sides rounded and anterior mar- 
gin straight. Apical angle about 75°. Anterior commissure broadly and deeply 
uniplicate in adults. Surface marked by concentric lines of growth and obscure 
radial lines. 
