CAMBRIAN FOSSILS. 451 



the high, nearly vertical area, aud the large delthyrium, partially closed 

 by a convex deltidinm. 



Formation and locality : Upper Cambrian, Gallatin terrane, Crowfoot 

 section, Gallatin Range ; also on the north slope of the Crowfoot Ridge, 

 on the south side of the Gallatin Valley, and on the divide between Panther 

 Creek and the Gallatin River. 



ORTHIS Dalman. 

 Orthis (!) rejinicha Wincliell. 



PI. LXI, figs. 3, 3a; PL LXII, figs. 1, la-d. 



Orthis remnicha Wiuchell, 18S6: Fourteenth Auu. Eept. Geol. aud Nat. Hist. Surv. 

 Minnesota, p. 317, PL II, fig. 7 . 



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 moderately 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 ears 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 devel- 

 oped on each valve, and divided by a rather large delthyrium. 



The ventral (pedicle) valve has in some specimens a shallow mesial 

 depression, and in some examples it is slightly flattened toward the cardinal 

 angles; beak small and curving down toward the hinge line, beyond which 

 it projects slightly. Dorsal (brachial) valve slightly less convex than the 

 ventral. Beak small, scarcely projecting beyond the hinge line. 



Surface marked by bifurcating, radiating' costse, that vary 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 specimens from the Park, as well as in those from 

 Texas and Wisconsin. In well-preserved specimens very fine, radiating, 

 raised striae occur both on the costae and on the intervening depressions. 

 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 the 

 upper Middle Cambrian horizon of Texas and the Park. 



The interior of tin- ventral (pedicle) valve shows a slightly raised, 



