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Show 1885.] DR. H. GADOW ON THE SPECIES OF RHEA. 311 americana possessing this foramen reduce its presence or absence to an unimportant individual variation. Regarding the vertebral column, the three species exhibit some Fig. 4. Skull of Rh. americana, \ natural size. Side view. Skull of Rh. macrorhyncha, £ natural size. Side view. Fig. 6. Skull of Rh. darwini, § natural size. Side view. highly interesting differences, which would be of great importance if w e could be certain that they are not due to individual variation. 21* |