5 so ON THE THEORY OF THE VERTEBRATE SKULL 



frontal, alisphenoid and petrosal bones, and so contributes more or 

 less largely to the completion of the cranial wall. 



But it has been most strangely forgotten that the relations of 

 the bone in question in birds, are by no means always those which 

 obtain in the ostrich. In the young of the commonest and most 

 accessible of domestic birds, in the chicken, the squamosal may be 

 readily seen to enter largely into the cranial wall ; a rhomboidal 

 portion of its anterior and internal surface being interposed in front 

 of the petrosal, between this bone, the parietal, the frontal, and the 

 alisphenoid (Sq. fig. 3). 



B.O 



Fig. 3. — Longitudinal section of the Skull of a young Chicken. 



There is therefore not a single relation (save the connexion of 

 the jugal) in which this bone does not resemble the squamosal of the 

 Mammalia — there is not one in which it does not differ from their 

 mastoid. 



The second bone applied externally to the cranium in the bird, 

 is that large and important structure, the os quadratum, which in- 

 tervenes between the petrosal and squamosal bones above, and the 

 articular portion of the lower jaw below ; which articulates with the 

 pterygoid internally, and with the quadratojugal externally, which 

 gives attachment to a part of the tympanic membrane, posteriorly, 

 and which is very generally termed the tympanic bone, from its 

 supposed homology with the bone so named in the Mammalia. The 

 resemblance to the tympanic bone, however, hardly extends beyond 

 its relation to the tympanic membrane ; for in no other of the 

 particulars mentioned above do the connexions of the two bones 

 correspond. The tympanic of the mammal does not articulate with 

 the lower jaw, nor with the pterygoid,'^ nor with the jugal or quad- 

 ratojugal. On the other hand, if the connexions of the tympanic 

 membrane were sufficient to determine the point, not only the quad- 



■* Though the pterygoid comes close to it in Monotyeinata. 



