230 



ON THE STRUCTUKE OF THE SKULL. 



and Reptiles, and is never known to articulate, by a moveable 

 joint, with the malleus, which, as we have seen, is the repre- 

 sentative of the os articular e of the mandible of the lower 

 Vertebrata. 



It is impossible, therefore, that the quadrate bone should be 

 the homologue of the tympanic of Mammalia. On the other 

 hand, it corresponds altogether with the quadrate bone of 

 Fishes, which is united in like manner with the pterygoid 

 arcade, and is similarly connected by a moveable joint with the 

 articular piece of the mandible ; and this quadrate bone of 

 Fishes is, I have endeavoured to show, the homologue of the 

 incus of the Mammalia. I make no question that, as Reichert 

 long ago asserted, the Bird's os quadratum and, therefore, that 

 of the Reptile, is the equivalent of the Mammalian incus. 



It is difficult to understand how any doubt can be enter- 

 tained as to the bone which is the homologue of the Mammalian 

 squamosal in Birds. Lying above the tympanic cavity, between 

 the parietal, frontal, and periotic bones, is a membrane bone 

 (Sq, Fig. 92, A) which corresponds with the Mammalian squa- 

 mosal, and with no other bone in the Mammalian skull. 



But if this be the Bird's squamosal, there is no difficulty in 

 determining that of any Reptile, the Crocodilia, Lacertilia, 

 Chelonia, and Ojjhidia all presenting a bone in a similar 

 position. It is this bone which, in most Ophidia (Fig. 92, C), 



Fig. 93. 



Fig. 93. — The skull of a Lizard (Cyclodus).—D D, Dentary piece of the lower jaw ; 



Q ', Os quadratum ; Sq, Squamosal. 



carries the quadratum as on a lever ; but, as Rathke has well 

 shown, the final position of the quadratum is a result of develop- 



