786 CAMBRIAN BRACmOPODA. 



EooRTHis EEMNiCHA (N. H. Wiiichell) . 

 Text figure 7, page 299; Plate XCI, figures 1, la-s; Plate XCII, figures 2, 2a-d, 3, 3a-e. 



Orthis remnicha N. H. Winohell, 1886, Fourteenth Ann. Kept. Geol. and Nat. Hist. Survey Minnesota, pp. 317-318, 

 PL II, fig. 7. (Described and discussed as a new species.) 



Orthisf remnicha Winohell, Walcott (in part), 1899, Mon, U. S. Geol. Survey, vol. 32, pt. 2, pp. 451^52, PI. LXI, 

 figs. 3 and 3a; PI. LXII, figs. 1, Ic-d (not figs, la and lb). (Described and discussed essentially as below. The 

 specimens represented by PI. LXI, figs. 3 and 3a, and PL LXII, figs. 1, Ic, and Id, are redrawn in this mono- 

 graph, PL XCII, figs. 2b, 2a, 2, 2d, and 2c, respectively. The specimens represented by PL LXII, figs, la and 

 lb, are now referred to Eoorthis iddingsi.) 



Orthis (Plectorthis) remnicha (Winchell), Walcott, 1905, Proc. U. S. Nat. Mus., vol. 28, pp. 268-269. (Described and 

 discussed essentially as below.) 



Plectorthis remnicha (Winchell), Geabau and Shimer, 1907, North American Index Fossils, vol. 1, p. 252. (Described.) 



Shell of medium size, usually slightly transverse, with an oblong, oval outline for the 

 ventral valve, and a subquadrate to semicircular outline for the dorsal valve. Valves moder- 

 ately convex, with an almost straight hinge line that varies in length from nearly the greatest 

 width of the shell to two-thirds the greatest width; cardinal angles varying from 90° or less in 

 the extreme forms, with extremities somewhat angular, to the other extreme, where they are 

 very obtuse and have the appearance of being almost rounded, their angle being not less than 

 120°. Cardinal area narrow but well developed on each valve, and divided by a rather large 

 delthyrium. 



The ventral valve has in some specimens a shallow mesial depression, and in some examples 

 it is flattened toward the cardinal angles; beak small and curving down toward the hinge line, 

 beyond which it projects slightly. Dorsal valve slightly less convex than the ventral; beak 

 small, scarcely projecting beyond the hinge line. 



Surface marked by bifurcating, radiating costse, that vary in number on shells of similar 

 size from 16 in the space of 5 mm. to 3 in the same space; this variation is shown in the speci- 

 mens from Texas, Wisconsin, and Wyoming. As the shell grew the ribs increased in number by 

 interpolation and by bifurcation from the sides of the larger ribs. In well-preserved specimens 

 very fine, radiating, raised striae occur both on the costse and on the intervening depressions . 

 (PI. XCII, fig. 2c). These are shown on the casts of the shells from the "St. Croix sandstone 

 of Winfield, Wisconsin, and on the larger shells from the limestones of Oklahoma and the 

 Yellowstone National Park. A very perfectly preserved fragment of the outer shell covering 

 the umbo and apex of a ventral valve from the limestone of the Reagan sandstone, Oklahoma- 

 has sharp ribs of varying size extending to the worn apex; the ribs are crossed by very fine, 

 threadlike striae and a few stronger lines of growth. 



The interior of the ventral valve shows a slightly raised tripartite pseudospondylium 

 beneath the umbo, which is the only trace of interior markings of this valve observed. The 

 interior of the dorsal valve has a slightly elevated area upon which occm-s a narrow, short 

 median septum; the crural plates are also well shown. In casts of the interior of shells from 

 the "St. Croix sandstone" of Wisconsin, the dental lamellae of the ventral valve are finely 

 shown, and in the dorsal valve the median septum and crural plates. 



Observations. — The shell is one of the most variable that occurs in the Cambrian fauna. 

 Its range of variation in all of the widely separated localities in which it occurs is such that one 

 would scarcely hesitate, if in possession only of the extremes, to identify two well-marked species. 

 The variation is not only in the radiating costse, but also in the general form of the shell. This 

 variation is expressed in three varieties that receive names, and others might be designated if 

 more minute variations were given consideration. 



There does not appear to be any system governing the number or size of the radiating ribs; 

 the variation on adult shells is equally great with that between young shells 3 to 4 mm. long 

 and adults. A series from Winfield, Wisconsin, representing Eoorthis remnicha winfieldensis, 

 is uniform in radial ribs and form from young shells 3 mm. long to the largest adults 23 mm. 

 long, and there are also shells that serve to unite the numerously and uniformly ribbed speci- 

 mens with the more variable Eoorthis remnicha. 



