698 CAMBKIAN BRAOHIOPODA. 



Similar forms occur in a compact, dark, bluish-gray limestone more than 200 miles to 

 the north in the Wasatch Mountains, east of Cache Valley, on the line of the Ute Peak section 

 of the Fortieth Parallel Survey. 



The specific name is derived from Ophir, the type locality. 



Formation and locality. — Middle Cambrian: (3e) Limestone less than 400 feet (121.9 m.) above the qvmtzitic 

 sandstones of the Cambrian, at Ophir, Oquirrh Range, Tooele County, Utah. 



(30n) About 3,750 feet (1,143 m.) above the Lower Cambrian and 650 feet (198 m.) below the Upper Cambrian, 

 in the shaly limestones forming Ic of the Weeks limestone [Walcott, 1908f, p. 175], on the north side of Weeks Canyon, 

 about 4 miles (6.4 km.) south of Marjum Pass; (30g) about 2,350 feet (716.3 m.) above the Lower Cambrian and 2,050 

 feet (624.8 m.) below the Upper Cambrian in the limestone forming Ic of the Marjum limestone; and (3x and 3y) about 

 2,200 feet (670.6 m.) above the Lower Cambrian and 2,200 feet (670.6 m.) below the Upper Cambrian in the shaly lime- 

 stones forming Id of the Marjum limestone [Walcott, 1908f, p. 180],' in the ridge east of Wheeler Amphitheater; all 

 in the House Range [Walcott, 1908f, PI. XIII], Millard County, Utah. 



(31z and 55t) About 350 feet (106.7 m.) above the Brigham quartzite in limestone of the Ute limestone [Walcott, 

 1908f, p. 7], on west side of road, 0.5 mile (0.8 km.) above the forks. Paradise Dry Canyon (locally known as East 

 Fork), east of Paradise, Cache County, Utah. 



ACROTRETA cf. OPIIIEENSIS Walcott. 



Numerous compressed specimens of shells about the size of Acrotreta opTiirensis Walcott 

 occur in strata of Middle Cambrian age at various localities in Utah. They resemble the latter 

 species in size, occasional strong lines of growth, position of the apex of the ventral valve, and 

 strong cardinal muscle scars in the dorsal valve; they differ in being much less convex, and in 

 having only a trace of shell substance; the removal of the shell by solution and the compression 

 in the shale accounts very largely for the differences. The stratigraphic position of the two 

 forms is about the same and they both lived in the eastern portion of the Cordilleran Middle 

 Cambrian Sea. 



Formation and locality. — Middle Cambrian: (32j) 625 feet (190.5 m.) above the top of the Cambrian quartzitic 

 sandstone in pinkish-colored very fine grained arenaceous shale, 2 miles (3.2 km.) southeast of Muskrat Spring, on 

 the northwest face of Grantsville Peak, Stansbury Range, Tooele County, Utah. 



(31v) 650 feet (198.1 m.) above the Lower Cambrian and 3,750 feet (1,143 m.) below the Upper Cambrian, in shales 

 at the top of the limestone forming la of the Howell formation [Walcott, 1908f, p. 182], northeast side of Dome Canyon, 

 about 4 miles (6.4 m.) west-southwest of Antelope Springs; (lly and 30g) about 2,350 feet (716.3 m.) above the Lower 

 Cambrian and 2,050 feet (624.8 m.) below the Upper Cambrian in Ic of the Marjum limestone [Walcott, 1908f, p. 180], 

 ridge east of Wheeler Amphitheater; and (31s) 490 feet (149.4 m.) above the Lower Cambrian and 3,925 feet (1,196.3 m.) 

 below the Upper Cambrian, in the pinkish argillaceous shale forming Id of the Howell formation [Walcott, 1908f, p. 

 182], south side of Dome Canyon about 1 mile (1.6 km.) below the divide, and 3 miles (4.8 km.) west-southwest of 

 Antelope Springs; all in the House Range [Walcott, 1908f, Pis. XIII and XVI], MUlard County, Utah. 



(31c) About 900 feet (274.3 m.) above the Brigham quartzite and 3,300 feet (1,005.8 m.) below the Upper Cam- 

 brian, in the limestone forming lb of the Ute limestone [Walcott, 1908f, p. 196], in Blacksmith Fork Canyon, about 10 

 miles (16.1 km.) east of Hyrum, Cache County; (32c) about 60 feet (18.3 m.) above the Brigham quartzite [Walcott, 

 1908a, p. 8], in green shale, 1 mile (1.6 km.) northwest of Geneva (Copenhagen), east of Brigham, Boxelder County; 

 and (30u) about 250 feet (76.2 m.) above the Cambrian quartzitic sandstones in sandy shales, 4 miles (6.4 km.) northwest 

 of Promontory Point (on the "Lucin cut-off" of the Union Pacific Railway), about halfway up west end of ridge, 

 north of Great Salt Lake, Boxelder County; all in Utah. 



Acrotreta ophirensis descendens Walcott. 



Plate LXXVIII, figures 1, la-c. 



Acrotreta ophirensis descendens Walcott, 1908, Smithsonian Misc. Coll., vol. 53, No. 3, p. 95, PI. IX, figs. 1 and la. 

 (Discussed as below as a new variety. Figs. 1 and la are copied in this monograph, PL LXXVIII, figs. 1 and Ic, 

 respectively.) 



In the Cambrian section of the House Range, Utah, the typical forms of Acrotreta opTiir- 

 ensis occur in the Marjum limestone; 1,570 feet higher in the section, in the Weeks limestone, 

 there are great numbers of a shell identical in many respects. This latter shell differs from the 

 typical form in being less convex in both the ventral and dorsal valves, and in having a more 

 pointed, less curved apex on the ventral valve. 



