BIRDS. 
2 9 2 
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.) 
