452 THE PALEONTOLOGY OF MINNESOTA. 
{Variety circularis. 
related to O. subequata and O. perveta Conrad. E. Billings states that* “it is distin- 
guished from O. subaquata by having a broad, shallow, mesial sinus in the front half 
of the ventral valve. * * * It may be that, by comparison with extensive 
series of western specimens, these three species might be united. I shall, for the 
present, keep them separate provisionally.” In Minnesota specimens occur which 
are identical with the O. gibbosa before us. The sinus toward the anterior margin 
is often profound in strongly convex specimens with angular sides, producing a fold 
in the dorsal valve which is distinctly limited laterally. For such shells we have 
retained the varietal name gibbosa. These and var. perveta at times merge into each 
other to such a degree that it is impossible to separate them. 
Formation and locality —Not uncommon in the Trentou shales at Minneapolis, Chatfield, Lanesboro 
and Cannon Falls, Minnesota; Decorah, Iowa. One specimen has been found in the ‘‘ Lower Blue beds” 
at Mineral Point, Wisconsin. ‘0 gibbosa occurs rarely in the Chazy limestone, island of Montreal; abund- 
antly, but badly preserved, at the Pallideau islands, lake Huron, in rocks which are either Chazy or Black 
River. At La Petite Chauditre rapids near Ottawa, and at the fourth chute of the Bonne-chtre, in the 
Black River limestone, and in the Trenton limestone at Bellville, Canada.” (Billings, Can. Nat. Geol. 
vol. iv, p. 435.) 
Collectors.—W. H. Scofield and the writers. 
Mus. Reg. Nos. 4032, 5510, 7969-7772. 
Variety crrcutaris N. H. Winchell. 
‘PLATE XXXIII, FIGS. 46 and 47. 
1880. Orthis cireularis N. H. WINCHELL. Highth Annual Report of the Geological and Natural 
History Survey of Minnesota, p. 66. 
Original description: “Shell subcircular, the greatest diameter being from just 
in front of one cardinal angle to the antero-lateral margin on the opposite side; 
hinge-line about one-half the greatest diameter; along the front margin is a very 
slight inclination toward the smaller valve, but the valves are otherwise uniformly 
convex; umbo of the receiving [ventral] valve prominent and full, but the beak low 
and arched over the cardinal area; the other valve less elevated in the umbo and 
the beak less prolonged, but slightly incurved over the hinge-line; the open foramen 
[delthyrium] of the receiving valve long and narrow, with an obtuse apex, but two 
or three times as wide at the base as at the top; surface marked by numerous fine 
rays which, bifurcating once or twice between the umbo and the free margin, are 
subequal at the middle of the front margin and number six or seven in the space of 
one line, two or three curving backward from the beak and terminating on the 
hinge area. These rays are crossed by fine concentric lines, only visible in fresh 
specimens and under a magnifier, and by distant dim growth bands, which latter 
begin on the umbo; diameter, about half an inch. Interior unknown.” 
*Oan. Nat. and Geol., vol. iv, p. 484, 1859. 
