33 2 CLASSIFICATION OF VERTEBRATES. 



of this keel was formerly used as the basis of division of the 

 birds into ratite and carinate groups. Its hinder edge may be 

 entire or with one or two deep notches, or there may be a fora- 

 men on either side. An episternum is apparently present in 

 the embryo. 



FlG. 323. Front and side views of sternum of common fowl, from Huxley. 



The skull of birds is noteworthy for its lightness, and for 

 the great extent to which the fusion of certain of the bones, 

 especially those of the cranial wall, has been carried, in all 

 except the dromaeognaths and some fossil forms. A temporal 

 fossa is present, but the supratemporal arcade is not complete. 

 Postfrontals and postorbitals are lacking. A jugal-quadrato- 

 jugal arch extends from the quadrate to the maxilla. The max- 

 illa is usually fixed ; but in some, as in the parrots, it is movable, 

 and in these cases its motion is transmitted by means of the 

 palato-pterygoid and the jugal-quadratojugal arches to the quad- 

 rate. The beak is largely made up of premaxillae, which are 

 fused and have a long frontal process. The orbits are placed 

 in front of the brain, and except in a few cases are separated by 

 a bony interorbital septum. Frequently there is an osseous, 

 sclerotic ring. The external nares are usually near the orbits. 



