292 



BIRDS. 



Skeleton. 



As the skeleton of Birds affords many important characters, 

 whereby the class is distinguished from Mammals, it is advisable to 

 enter at once upon its consideration. In the first place, the skull of a bird, as 

 shown in the figures given later on in the chapter, differs from that of a mammal 



SKELETON OF PARROT AND SKULL OF COCKATOO. 



in that it is attached to the first joint of the backbone by a single knob or condyle, 

 instead of by two such condyles. Secondly, each half of the lower jaw is composed 

 of several pieces, instead of but one ; and instead of the lower jaw articulat- 

 ing directly with what is 

 known as the squamosal region 

 of the brain-case, it does so by 

 the intervention of a separate 

 bone, termed, from its form, 

 the quadrate, the position of 

 which is indicated in the 

 accompanying figure. It may 

 be mentioned here that in all 

 existing Birds both jaws are 

 encased in horn, and are devoid 

 of teeth ; while the two halves 

 of the lower jaw are completely soldered together by bone at their junction, or 

 symphysis. Certain extinct Birds had, however, a full series of teeth, and the two 

 halves of the lower jaw separate. 



SIDE VIEW OF SKULL OF TEAL, WITH THE LOWER JAW DISPLACED. 



The bone immediately to the left of the one marked Pt is the 

 quadrate. (From Huxley, Proc. Zool. Soc., 1867.) 



