The taxonomy of the genus Camarasaurus is discussed. On the basis of the material examined only two species (supremus Cope and lentus Marsh) are considered valid. An additional braincase of the problematical Camarasaurus agilis (Marsh) is briefly described. The braincase of Apatosaurus ajax Marsh is briefly described from the type specimen and a specimen in the American Museum of Natural History. The braincase and the internal aspects of several skull bones of C. lentus (Marsh) are described in detail and figured. The mode of articulation of the lower jaw with the quadrate and the type of wear of the teeth suggest that this herbivorous dinosaur chewed its food in the manner of the herbivorous mammals. The order of tooth replacement in herbivorous dinosaurs is discussed.