The author discusses the evidence afforded by the plants, invertebrates, fishes, amphibians, birds, reptiles (especially the dinosaurs), and mammals found in the Morrison beds of western N. America as to the age of that formation. The Morrison dinosaurs are compared with those found in the Potomac Formation of the eastern U. S. A., in the Jurassic and Cretaceous beds of Europe, and in the Tendaguru beds of eastern Africa; and concludes that the Morrison faunas probably lived in Kimmeridgian or Portlandian, Neo-Jurassic, times. Bibliography.