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 diffi- 

 culty dilated from the mouth, but pressure on the thorax (repre- 

 senting 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 Emu bears out the Reptilian affi- 

 nities of the StruthionidcB which Huxley* and Parker f 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 Thomas H. Huxley^ 

 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 repre- 

 sent 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 aftei- these have become anchy- 

 losed with the supraoccipital and exoccipital. 



.5. The mcus and malleus are not subservient to the function of 

 hearing as ossicida auditus. 



* Elements of CoTiip. Aiiat. 186 1, i. (and unpublished Iluiiteriau Lectures, 1807). 

 t Philosophical Transactions, 186G, pp. 113-183. 



