436 THE PALEONTOLOGY OF MINNESOTA. 
[Orthis (Plectorthis) pliciitella. 
Orthis (Plectorthis) plicatella Hall. 
PLATE XXXin, FIGS. 5-7. 
18-J7. Orthis plicatella Hall. Palajontology of New York, vol. i, p. 122, pL xxxii, fig. 9. 
■?186'j. Orthis plicatella BiixiXGS. Geology of Canada, p. 165, fig 145. 
1873. O-lhis plicatella Meek. Pahvontology of Ohio, vol. 1, p. lOS, pi. a^ii, flg. 7. 
1875. Orthis pbc delta MiLLEii. Cincinnati Qa irterly Journal of Science, vol. ii, p. 30. 
1892. Plectorthis plicatella Hall. PaUt'ontology of New York, vol. viii, pt. i, p. 221, pi. v, tigs. 18-20. 
Original description: "Broadly semioval, nearly equivalve, length and breadth 
about as three to four; surface marked by strong, radiating plicae, which are usually 
simple, about twenty to twenty-eight on each valve, crossed by simple, elevated con- 
centric lines, which are more distinct in the depressions between the costae, and 
often obscure or obsolete upon their exposed surfaces; valves nearly equally convex, 
without sensible depression or elevation on either one, meeting on the edges in a 
straight line; cardinal lines not extending beyond the width of the shell; area nar- 
row; dorsal foramen not extending to the beak." 
This well known species originates in the lower portion of the Trenton group 
of New York and is also found in the inferior strata of the Galena formation in 
Wisconsin and Minnesota, attaining its greatest development in numbers and varia- 
tions in the Cincinnati group, at a horizon about 300 to 350 feet above low water 
in the Ohio river. 
Orthis plicatella is rather a rare species in the Trenton of New York, and Galena 
of Wisconsin and Minnesota. It attains a larger growth and is fairly constant in its 
simple plications and has a more depressed and furrowed dorsal umbo than in speci- 
mens from the Cincinnati group. The size and number of plications in the specimens 
from Cincinnati are very variable, several forms having been described as distinct 
species. These are 0. Jissicosta, 0. dichotoma Hall,* and 0. triplicatella Meek.** In 
large collections of these forms, however, it is often difficult to know where to draw 
the line between them, and the above names can only be used to indicate the tran- 
sition from the simple undivided 0. plicatella to 0. dichotoma with its numerous 
interpolated smaller striae. 
In the Cincinnati group of the Northwest, 0. plicatella is not known to occur, 
but 0. tchiifieldi Winchell and 0. hankakensis McChesney take its place. These species 
have attained a much larger size than 0. plicatella. 
Formation and loenlity.—'RaTe in the Trenton limestone of Middleville and Watertown, New York, 
and Biirgin, Kentucky. One individual has been discovered in the Galena shales at St. Paul; it has also 
been obtained at Kenyon, Cannon Kails and elsewhere in Goodhue county, Minnesota. Prof. Whitfield 
reports it from about tbe same horizon in Wisconsin. Very common in the Cincinnati group around 
Cincinnati, Ohio. 
Collectors.— \Y. H. Scofleld, A. D. Meeds and the writers. 
Mus. Reg. Nos. 7765, 7766. 
* Pal. N Y., vol. 1. pp. IJl, 125. pi xxxi i, figs. 7, 13, 1847. 
♦* Pal. Obio, vol. 1. p. 101), pi. vill, fig . 8. 1872. 
