452 THE PALEONTOLOGY OF MINNESOTA. 



(.Variety iMrmlarts. 



related to 0. subwquata and 0. perveta Conrad. E. Billings states that* "it is distin- 

 guished from 0. subcp.quata 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 0. 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 Trcntou shales at Minneapolis, Chatfleld, Lanesboro 

 and Cannon Falls, Minnesota; Decorah, Iowa. One specimen has been found in the "Lower Blue beds" 

 at Mineral Point, Wisconsin. " 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 

 Eiver. At La Petite Chaudtfre rapids near Ottawa, and at the fourth chute of the Bonne-chf're, in the 

 Black River limestone, and in the Trenton limestone at Bellville, Canada." (Billings. Can. Nat. Gool. 

 vol. iv, p. 435.) 



Collectors. W. H. Scofleld and the writers. 

 Mus. Reg. Nos. 4032, 5510. 7969-7772. 



Variety CIRCULABIS N. H. Winchell. 



PLATE XXXIII, FIGS. 46 ami 47. 



1880. Orthis cireularis N. H. WINCHELL. Eighth 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." 



*Can. Nat. and Geol., vol. IT, p. 434, 1859. 



