0B0LID3:. 457 



young by figures le and If. The shells found near Helena, Montana, commonly occur as casts 

 in a siliceous shale and have the form of and appear to be specifically identical with the species 

 in the Wasatch and Oquiri-h mountains. A few specimens preserve the shell and show it to 

 be thick and stronglj^ laminated toward the front. 



The form from the Bighorn Mountains of Wyoming appears to be identical with this 

 species, except that the surface ornamentation is coarser and much more distinctly outlined by 

 the zigzag, irregular, transverse strife. The strength of the strife and the size of the more or 

 less rhombic interspaces range from near those of 0. ( IF.) ella to more than double their size, 

 the increase in the size of the interspaces being made more prominent by the deposition of 

 what appears to be chalcedony on the outer surface or a replacement of the shell by chalcedony. 

 The form of the valves and the position of the central and anterior lateral muscle scars in the 

 dorsal valve are essentially the same as in 0. (TF.) ella. 



On Gordon Creek in Montana this species occurs in association with the Albertella JieleTia 

 fauna [Walcott, 1908f, p. 202], which is referred to the highest zone of the Lower Cambrian. 

 Four small ventral valves were found that have the form and surface characteristic of Obolus 

 (Westo7iia) ella. 



Oholus ( Westonia) ella has a wide geographic distribution in the Cbrdilleran and southern 

 Appalachian regions. It also has a vertical range from the summit of the Lower Cambrian 

 in Montana and British Columbia to 250 feet up in the Upper Cambrian. 



Formation and locality. — Upper Cambrian: (4y and 54t) Limestone of the St. Charles formation [Walcott, 1908a, 

 p. 6], about 250 feet (76 m.) above the Jliddle Cambrian on the north side of Twomile Canyon, near its mouth, 2 miles 

 (3.2 km.) southeast of Malade, Oneida County, Idaho. 



(54e) About 200 feet (61 m.) above the Middle Cambrian and 1,025 feet (212.4 m.) below the top of the Upper 

 Cambrian in limestones forming 3 of the St. Charles formation [Walcott, 1908f, p. 193], in Blacksmith Fork Canyon, 

 about 10 miles (16.1 km.) east of Hyrum, Cache County, Utah. 



Middle Cambrian: (57g) About 1,700 feet (518 m.) above the Lower Cambrian and 3,250 feet (991 m.) below the 

 Upper Cambrian, in the siliceous shales forming 2d of the Stephen formation [Walcott, 1908f, p. 211], on Mount Bos- 

 worth, north of the Canadian Pacific Kailway between Hector and Stephen, on the Continental Divide, between 

 British Columbia and Alberta, Canada. 



(5f) Limestone interbedded in the Wolsey shale [Weed, 1900, p. 285], in Meagher County, on the road to Wolsey, 

 about 4 miles (6.4 km.) south of the divide at the head of Sawmill Creek, and 11 miles (17.7 km.) south of Neihart, 

 Little Belt Mountains quadrangle (U. S. Geol. Survey), Cascade County; (340a) shales near Helena, Lewis and Clark 

 County; (302s) dark siliceous shale in a quarry in Last Chance Gulch, south slope of Mount Helena, 1.5 miles (2.4 km.) 

 south of Helena, Lewis and Clark County; (4u) shales in the Gallatin formation of Peale [1893, PL IV], northwest side of 

 canyon 0.5 mile (0.8 km.) south of Helena, Lewis and Clark County; (9m) shale 4 miles (6.4 km.) above Walker's ranch, 

 on North Fork of Dearborn River, in the eastern part of the Lewis and Clark National Forest, Lewis and Clark County; 

 (iq') limestone about 310 feet (94.5 m.) above the unconformable base of the Cambrian and 185 feet (56.4 m.) above 

 the top of the quartzitic sandstones in a shale which corresponds in stratigraphic position to the upper part of shale 

 No. 6 of the Dearborn River section [Walcott, 1908f, p. 202], on the ridge between Gordon and Youngs creeks, about 

 halfway between Gordon Mountain and Cardinal Peak, Ovando quadrangle (U. S. Geol. Survey), Powell County; 

 (4k) about 725 feet (221 m.) above the base of the Cambrian in the shales of the Gallatin formation of Peale [1893, PI. 

 IV], on the north side of West Gallatin (Gallatin) River, northeast of Logan, Threeforks quadrangle (U. S. Geol. Survey), 

 Gallatin County; and (4g) about 325 feet (99.1 m.) above the base of the Cambrian in the Flathead shales of Peale 

 [1893, p. 21], 1 mile (1.6 km.) north of the junction of East Gallatin and West Gallatin (Gallatin) rivers, 4 miles (6.4 

 km.) east-northeast of Logan, Threeforks quadrangle (U. S. Geol. Sm'vey), Gallatin County; all in Montana. 



(6h and 55d) About 2,000 feet (609.6 m.) above the Cambrian quartzitic beds in a shale which is probably to be 

 refeiTed to the Bloomington formation [Walcott, 1908a, p. 7], on the south side of Twomile Canyon, near its mouth, 

 2 miles (3.2 km.) southeast of Malade, Oneida County; and (55c) Spence shale member of the Ute limestone [Walcott, 

 1908a, p. 8], about 50 feet (15.2 m.) above the Brigham quartzite and 2,755 feet (839.7 m.) below the Upper Cambrian, 

 in a ravine running up into Danish Flat from Mill Canyon, about 6 miles (9.6 km.) west-southwest of Liberty and 15 

 miles (24.2 km.) west of Montpelier, Bear Lake County; both in Idaho. 



(171a) Sandstone in Big Goose Creek Canyon, in the Bighorn Mountains, west of Sheridan, Sheridan County; 

 and (4n) limestone about 325 feet (99.1 m.) above the unconformable base of the Cambrian in divide at the head of 

 Jackson Creek (locally known as Sheep Creek), a creek flowing into Jackson Lake about 0.5 mile (0.8 km.) south of its 

 northwestern comer, Teton Mountains, Grand Teton quadrangle (U. S. Geol. Survey), Uinta County; both in Wyoming. 



(3g) Shales on the river bank 250 feet (76.2 m.) above the west end of the Denver and Rio Grande Railroad 

 tunnel, Glenwood Springs, Garfield County, Colorado. 



