In his work on the Cambrian and related Ordovician Brachiopoda the writer has had access to approximately 1,350 of the Cambrian fossil localities represented in the collections of the United States National Museum,2 from about 72 per cent of which, or 975, brachiopods have been identified. All of this material and all known references to the occurrence of Cambrian Brachiopoda throughout the world were utilized in this study of the nature of their inclosing sediments, an investigation which was first suggested by Professor Schuchert in a letter to Mr. Walcott, and seemed to be justified by the abundance of the material, its variety, and the magnitude of its geographic range. The present report is based on the study of 44 genera, 15 subgenera, 477 species, and 59 varieties of Cambrian Brachiopoda, and 3 genera, 1 subgenus, 42 species, and 1 variety of exclusively Ordovician Brachiopoda from 1,460 localities within . . .