420 THE PALEONTOLOGY OF MINNESOTA. 
(Dinorthis. 
O. disparilis Conrad proves to be the young of 0. se) as has been suggested 
by Hall and Whitfield.* 
O. davidsoni de Verneuil,+ of the Silurian of America, England and Scotland, is 
another closely related species, but can be distinguished from 0. tricenaria by its 
more convex cardinal area and the strongly elevated subangular costa. The costv 
also have a few remote oblique perforations in the shell substance, a feature never 
seen in O. tricenaria. 
Formation and locality.—This widely-distributed and characteristic Trenton fossil is often smaller 
in size in the Northwest than in either the eastern or southern exposures of the horizon. It occurs com- 
monly as natural casts in the Trenton limestone at Minneapolis, St. Paul and Cannon Falls; but in the 
Trenton shales good shells are not rare at. Minneapolis, St. Paul, Cannon Falls, Fountain, near Lanesboro, 
Byota, Preston, and near Caledonia, Minnesota; also at Decorah and McGregor, Iowa. In Wisconsin it is 
very abundant, and is a characteristic fossil of the ‘‘ Lower Blue beds” at Beloit, Janesville and Mineral 
Point; at the last locality it was also collected in a siliceous condition near the base of the ‘Upper Buff 
beds.” It has also been collected by one of the writers in the Trenton formation at Dixon, Illinois; High 
Bridge and Curdsville, Kentucky; and Watertown, New York. Near Ottawa and Montreal, Canada: Min- 
gan islands; Eureka and White Pine districts, Nevada. In Goodhue county, Minnesota, this species is 
known to extend upwards for at least fifty feet in the Galena formation at several exposures south of 
Cannon Falls. 
Collectors.—Miss C. S. Seymour, C. L. Herrick, H. V. Winchell, J. C. Kassube, W. H. Scofield, E. O- 
Ulrich and the writers. - 
Mus. Reg. Nos. 372, 666, 668, 2191, 3509, 4034, 5057, 5091, 5130, 5150, 5582, 6802, 7795-7810, 7916. 
Subgenus DINORTHIS, Hall. 
1892. Dinorthis, HALL. Paleontology of New York, vol. viii, pt. i, p. 195. 
1892. Plesiomys, HALL. Ibidem, p. 196. 
Original description: “This group of shells, in its most characteristic examples, 
presents a reversal of the relative convexity of the valves as seen in Orthis calli- 
gramma. The pedicle valve, elevated at the umbo, becomes gradually depressed as 
growth advances, and in the mature condition is flat or gently concave over the 
pallial region. The brachial valve, on the other hand, is eminently convex. The 
surface is marked by strong [and fine], simple, rarely bifurcating costa, as in 0. 
calligramma. The cardinal area of the pedicle valve is well developed, but not 
greatly elevated. In the interior the dental lamella are prominently developed 
and are extended around a subquadrate muscular area, the strength of which 
apparently depends upon the age and thickness of the shell. The three pairs of 
impressions may often be distinguished; the elongate adductors occupying a central 
position and separated by a faint median ridge, the diductors forming large ante- 
lateral expansions enclosing the adductors; the adjustors lie outside and behind 
these [pedicle muscles not always discernible]. Occasionally, in Orthis pectinella, 
there is again seen the gradual closing of the delthyrium of the pedicle valve by an 
*Pal. New York, vol. i, p. 20, pl. Iv bis, fig. 4, 1847. 
+ Bull. Soc. Geol. de France, sec. ser., vol. v, p. 341, pl. rv, fig. 9, 1848. 
