Mr. T. H. Huxley on the Theory of the Vertebrate Skull. 423 



eventually developes a condyle which comes into contact, and arti- 

 culates, with the squamosal. In the bird, on the contrary, the 

 ramus of the jaw unites with the ossified proximal end of Meckel's 

 cartilage, which becomes anchylosed with the ramus ; but retaining 

 its moveable connexion with the quadratum (or representative of the 

 incus), receives the name of the articular piece of the jaw. The 

 rest of Meckel's cartilage disappears. 



Thus the primitive composition of the mandibular cartilaginous 

 arch is the same in the bird as in the mammal ; in each, the arch 

 becomes subdivided into an incudal and a Meckelian portion ; 

 in each, the incudal and the adjacent extremity of the Meckelian 

 cartilage ossify, while the rest of the cartilaginous arch disappears 

 and is replaced by a bony ramus deposited round it. But from this 

 point the mammal and the bird diverge. In the former, the in- 

 cudal and Meckelian elements are so completely applied to the pur- 

 poses of the organ of hearing, that they are no longer capable of 

 supporting the ramus, which eventually comes into contact with the 

 squamosal bone. In the latter, they only subserve audition so far 

 as they help to support the tympanic membrane, their predominant 

 function being the support of the jaw. 



The tympanic bone of every mammal is, at first, a flat, thin, 

 curved plate of osseous matter, which appears on the outer side of 

 the proximal end of Meckel's cartilage, but is as completely indepen- 

 dent of it as is the ramus of the jaw of the rest of that cartilage. 

 In most birds it has no bony representative. 



It is clear, then, as Professor Goodsir* has particularly stated, that 

 the OS quadratum of the bird is the homologue of the incus of the 

 mammal, and has nothing to do with the tympanic bone ; while the 

 apparently missing malleus of the mammal is to be found in the 

 OS articulare of the lower jaw of the bird. 



It would lead me too far were I to pursue the comparison of the 

 bird's skull with that of the mammal further. But sufficient has 

 been said, I trust, to prove that, so far as the cranium proper is con- 

 cerned, there is the most w^onderful harmony in the structure of the 

 two, not a part existing in the one which is not readily discoverable 

 in the same position, and performing the same essential functions, in 

 the other. I have the more willingly occupied a considerable time 

 in the demonstration of this great fact, because it must be universally 

 admitted that the bones which I have termed petrous, squamosal, 

 mastoid, quadratum, articulare in the bird, are the homologues of 

 particular bones in other oviparous Vertebrata, and consequently, if 

 these determinations are correct in the bird, their extension to the 

 other Ovipara is a logical necessity. But the determination of these 

 bones throughout the vertebrate series is the keystone of every theory 

 of the skull — it is the point upon which all further reasoning must 

 turn ; and therefore it is to them, in considering the skulls of the 

 other Ovipara, that I shall more particularly confine myself. 



* Reichert, however, had already clearly declared this important homology in 

 liis ' Entwickelungsgesehichte des Kopfes,' p. 195. 



