420 THE PALEONTOLOGY OP MINNESOTA. 



IDinorthis. 



0. disparilis Conrad proves to be the young of 0. tricenaria, as has been suggested 

 by Hall and Whitfield.* 



0. 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 costae. The costse 

 also have a few remote oblique perforations in the shell substance, a feature never 

 seen in 0. tricenaria. 



Formation and locality. This widely-distributed and characteristic Trenton fossil is often smaller 

 in size iu 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, 

 Eyota, 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 flfty 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. Scofleld, E. O- 

 Ulrich and the writers. 



Mvs. Reg. Nos. 372, 666, 668, 2191, 3509, 4034, 5057, 5091, 5130, 5150, 5582, 6802, 7795-7810, 7916. 



Subgenus DINORTHIS, Hall. 



1892. Dinorthis, HALL. Palaeontology of New York, vol. viii, pt. i, p. 195. 

 1892. Plcesiomys, 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 costaa, as in 0. 

 calligramma. The cardinal area of the pedicle valve is well developed, but not 

 greatly elevated. In the interior the dental lamellae 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 adjusters 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, pi. iv Ms, fig. 4, 1847. 



+ Bnll. Soc. Geol. de France, sec. sen, vol. v, p. 341, pi. iv, fig. 9, 1848. 



