436 THE PALEONTOLOGY OF MINNESOTA. 



[Orthis (Plectorthis) plicatella. 



ORTHIS (PLECTORTHIS) PLICATELLA. Hall. 



PLATE XXXIII, FIGS. 5-7. 



18-47. Orthis plicatella HALL. Palaeontology of New York, vol. i, p. 122, pi. xxxn, fig. 9. 



?IS(n. Orthis plicntelln BILLINGS. Geology of Canada, p. 165, fig 145. 



lt<73. Orthis plicaietla MKEK. Palaeontology of Ohio, vol. i, p. 10S, pi. viti, fig. 7. 



1875. Orthis pl'C drlla MILLER. Cincinnati Qu irterly Journal of Science, vol. ii, p. 30. 



1892. Plectorthis plicatella HALL. Palaeontology of New York, vol. viii, pt. i. p. 221, pi. v, figs. 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. wliitfieldi Winchell and 0. kankakensis McChesney take its place. These species 

 have attained a much larger size than 0. plicatella. 



Formation and loc>tlity.~Rare in the Trenton limestone of Middleville anrl Watertown, New York, 

 and Burgin, 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 Good hue county, Minnesota. Prof. Whittled 

 reports it from about the same horizon in Wisconsin. Very common in the Cincinnati group around 

 Cincinnati, Ohio. 



Collectors. W. H. Scofleld, A. D. Meeds and the writers. 

 Mus, Reg. Nos. 7765, 7766. 



* Pal. N. Y., vol. i, pp. 121, 125. pi xxxii. figs. 7, 13, 1817. 

 "Pal. Ohio, vol. 1. p. 109, pi. vui, fig. 8. 18."2. 



