U,t,, is (Helena) boreal,..] BRAOHIOPODA. 433 



ORTHIS (HEBEBTELLA) BOREALIS Billings. 



FIG. 33. Orthis (Hebertella) borealis Billings, a, dorsal; b, ventral, and c, profile view of a variety 

 of this species. Upper part of the Galena shales near Wykoff, Minnesota. Collection of E. O. Ulrich. 



1859. Orthis borealis BILLINGS. Canadian Naturalist and Geologist, voi. iv, p. 436, flg. 14. 



1863. Orthis borealis BILLINGS. Geology of Canada, p. 129, flg. 56; p 167, flg. 148. 



1873. Orthis borealis MEEK Palaeontology of Ohio, vol. i, p. 101, pi. vm, flg. 4. 



1875. Orthis borealis MILLER. Cincinnati Q larterly Journal of Science. vo 2, p. 23. 



1889. Orthis borealis NETTELROTH. Kentucky Fossil Shells, p. 36. pi. xxxiv, flgs. 14-20. 



1892. Orthis (Hebertella) borealis HALL. Palaeontology of New York, vol. viii, pt. i, p. 222. 



The Minnesota specimens of this species are closely related to Orthis plicatella 

 of the Galena horizon, so that a detailed description is not necessary. They can be 

 distinguished from each other by the fold and sinus, in 0. borealis, being on the 

 dorsal and ventral valves respectively, while in 0, plicatella the conditions are the 

 reverse. These differences are not so pronounced as in specimens from the Ohio 

 valley, where 0. borealis usually has, in addition, a much deeper dorsal than ventral 

 valve. The latter character is one of the chief distinctions between Prof. Hall's 

 subgenera Hebertella and Plectorthis, and is developed at its maximum in 0. sinuata 

 Hall. From the preceding it is readilly seen that these subgenera probably had 

 their origin in the Chazy formation, and there and in the Trenton are not easily 

 distinguishable, but in the Hudson River group of the Ohio valley Hebertella is one of 

 the characteristic markers of that horizon, there reaching its maximum of growth, 

 diversity and number of individuals. Plectorthis also attains its maximum in the 

 Hudson River group, the tendency in the Cincinnati group being towards diminished 

 size, but greater numerical strength and irregularity of plications, while in the north- 

 west the radical form developed into other distinct, large, somewhat localized species. 

 Hebertella persists into the Upper Silurian, while Plectorthis, which probably origin- 

 ated first, became extinct with the Hudson River group. 



Formation and locality. Rare in the upper Clitambonites horizon of the Galena shales near Wykoff, 

 Cannon Falls and top of West St. Paul bluffs, in Minnesota. Prof. Whitflekl mentions its occurren-e in 

 Wisconsin. Very common near the top of the Trenton limestone at Frankfort, Burgin, Lexington and 

 elsewhere in central Kentucky. Near Nashville, Tennessee. In Canada Billings cites it from the Chazy 

 to the Trenton at Caughnawaga, St. Genevieve, Isle Bizard and Cornwall. 



Collectors. E. O. Ulrich and C. Schuchert. 



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