THE SKULLS OF REPTILIA AND AVES. 229 



into a beak, the internasal part of the basi-facial axis acquires 

 a considerable size, and becomes the subject of a great variety 

 of ossifications, which, in many Birds, are so arranged as to 

 allow the anterior part of the cranio-facial axis to be moveable on 

 its posterior part. In many Lizards, on the other hand, the an- 

 terior part of the cranium is rendered moveable upon the pos- 

 terior in another way. The cranio-facial axis in front of the basi- 

 sphenoid is cartilaginous, and consequently slightly flexible, 

 w T hile the roof of the skull between the parietals, the supra- 

 occipital and the periotic bones is merely membranous (f, Fig. 

 90, A) ; hence, the front part of the skull is capable of being 

 slightly raised or depressed, in a vertical plane, upon the 

 posterior part. 



Next to the pro-otic, the squamosal and the quadrate bones 

 of Birds and Reptiles have been the subject of the greatest 

 amount of controversy among morphologists. 



The bone which was originally called " os quadratum ' 

 is that moveable facial bone of the Bird (Qu, Fig. 92, A) which 

 is articulated, above, with the outer side of the periotic capsule, 

 and especially with the pro-otic bone, and below with the 

 os articulare of the mandible, while, internally and anteriorly, it 

 is connected with the pterygoid. In the Crocodilia (Fig. 92, B) 

 and Chelonia, a bone, admitted by all to be the homologue of 

 this, is attached immoveably in the same region : in most 

 Lacertilia (Fig. 93) it is moveable, and remains connected 

 with the produced extremity of the pro-otic bone ; but, in most 

 Ojyhidia (Fig. 92, C) its proximal end is thrust out from the 

 skull upon the extremity of another bone. However, its 

 homology with the quadrate of the Bird is not affected by this 

 circumstance. 



With what bone in the human skull does this correspond ? 

 Cuvier identified it with the tympanic of Man, and his interpre- 

 tation has been generally accepted ; but the tympanic is always 

 a membrane bone, whereas this is always a cartilage bone. The 

 tympanic directly supports the tympanic membrane, while this 

 bone sometimes gives no direct attachment to the tympanic 

 membrane at all. The tympanic of Mammals again becomes 

 smallest in those Mammalia which most nearly approach Birds 



