ON THE THEORY OF THE VERTEBRATE SKULL 553' 



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 carti- 

 lage, 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 incudal 

 and Meckelian elements are so completely applied to the purposes 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 independ- 

 ent 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 concerned, 

 there is the most wonderful 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 



1 See Note HL 



^ Reichert, however, had already clearly declared this important homology in his 

 ' Entwickelungsgeschichte des Kopfes,' p. 195. 



