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Show 1867.] PROF. HUXLEY ON THE CLASSIFICATION OF BIRDS. 415 its free or distal extremity. The communicating aperture also was roundish, and placed high up in the trachea, close to the larynx, not slitlike or so low down as in the Emu. It is also worth while here to note that in the dead Chameleon the tracheal sac was with difficulty dilated from the mouth, but pressure on the thorax (representing forced expiration) distended it with ease, a fact corroborative of its mode of action in the living Emu. With these homological remarks I have said enough to show that the accessory tracheal pouch in the E m u bears out the Reptilian affinities of the Struthionidce which Huxley* and Parkerf more clearly bring out from observations on their osteological development. 4. On the Classification of Birds; and on the Taxonomic Value of the Modifications of certain of the Cranial Bones observable in that Class. By T H O M A S H . H U X L E Y , F.R.S., V.P.Z.S. The members of the class AVES so nearly approach the REPTILIA in all the essential and fundamental points of their structure, that the phrase "Birds are greatly modified Reptiles" would hardly be an exaggerated expression of the closeness of that resemblance. In perfect strictness, no doubt, it is true that Birds are no more modified Reptiles than Reptiles are modified Birds, the reptilian and the ornithic types being both, in reality, somewhat different superstructures raised upon one and the same ground-plan; but it is also true that some Reptiles deviate so very much less from that ground-plan than any Bird does, that they might be taken to represent that which is common to both classes, without any serious error. A Lizard is not very far from being the centre of the circle, the periphery of which is occupied by Chelonia, Ichthyosauria, Plesio-sauria, Pterosauria, and Aves. That the association of Birds with Reptiles into one primary group of the Vertebrata, the SAUROPSIDA, which I have proposed elsewhere, is not a mere fancy, but that the necessity of such a step is as plain and demonstrable as any position in taxonomy can be, appears to me to be proved by an enumeration of the principal points in which Aves and Reptilia agree with one another and differ from Mammalia. 1. They are devoid of hair. 2. The centra of their vertebrae have no epiphyses. 3. Their skulls have single occipital condyles. 4. The prootic bone either remains distinct throughout life, or unites with the epiotic and opisthotic after these have become anchylosed with the supraoccipital and exoccipital. 5. The incus and malleus are not subservient to the function of hearing as ossicula auditus. * Elements of Comp. Anat. 1864, i. (and unpublished Hunterian Lectures, 1867). f Philosophical Transactions, 1866, pp. 113-183. |