NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 3OI 



Obolus {Wcstonia) clla (Hall and Whitfield) — Continued. pagi: 



Fig. 9. (X^-) A distorted dorsal valve associated with the specimen 

 represented by fig. 7, U. S. National Museum, Catalogue 

 No. 62549. 



Figs. 7, 8 and 9 are from the Middle Cambrian: (302s) Park 

 shale, on northwest side of Grizzly Gulch, about 1.5 miles southwest 

 of the city of Helena. 



10. (X 10.) Enlargement by photography of the surface of a 

 specimen preserved in a shale where the exterior surface 

 does not adhere to the matrix. Where the surface clings to 

 the matrix, the specimens have the appearance of those 

 illustrated by figs. 7 and 9. 



Fig. 10 is of a specimen from locality 4g, Middle Cambrian: 

 about 325 feet (99.1 m.) above the base of the Cambrian, in the 

 Flathead shales of Peale, i mile (1.6 km.) north of the junction of 

 East and West Gallatin Rivers, Gallatin County, Montana. 



U. S. National Museum, Catalogue No. 51685c. Specimen figured 

 by Walcott, fig. i 0, Plate 47, Monogr. 51, U. S. Geol. Surv., 1912. 



