The author, after quoting from the works of writers on Ichthyology and Comparative Anatomy their descriptions of the vertebral columns of cartilaginous fishes, proceeded to point out the existence of an osseous structure in the vertebræ of the Plagiastomi, or Rays and Sharks. In thse animals the essential portion of each vertebra consists of a double cup or saucer-shaped osseous piece, resembling in form and structure the double cup-shaped vertebræ of osseous fishes, being, like them, composed of concentric rings of osseous matter.