The book details the fossilized life forms discovered in North America during the Cambrian period. Charles D. Walcott, a prominent paleontologist, presents his findings on the rich faunas found in various geological formations. The work focuses on the classification, morphology, and paleoecology of these ancient organisms, drawing upon extensive fieldwork and specimen analysis. It aims to provide a comprehensive overview of the Cambrian life that existed on the North American continent.

This seminal work by Charles D. Walcott is regarded as a foundational text in North American Cambrian paleontology. Its detailed descriptions and extensive cataloging of fossils provided crucial data for understanding early life forms. The book's meticulous approach and the breadth of its coverage established a benchmark for subsequent research in the field. It remains a significant reference for paleontologists studying the Cambrian explosion and its North American manifestations.