The work of investigators, during the past decade, upon the Cambrian sediments and fossils of central and southern Montana and northwestern Wyoming, has shown conclusively that further productive stratigraphic work could not be done until the exact meaning of Weed’s original formations is made known. Four field seasons in the Rocky Mountains of northwestern Montana, and several years’ study of the faunas, plus two months in the summer of 1934 comparing the new genera and species directly with the type Cambrian fossils in the United States National Museum, forced the writer to the same conclusion. Examination of the geologic literature shows that the original definitions of formation names were not adhered to by later writers, and that different workers have not referred to the same strata by the same names. This confusion of terms is reflected in the erroneous assignment of certain species to widely different formations. . . .