BIRDS. 381 



upon a long and slender neck. The fore-limbs were very short, 

 but the hind-limbs were long and like those of Birds. The 

 proximal portion of the tarsus resembled that of Birds in being 

 anchylosed to the lower end of the tibia ; but the distal portion 

 of the tarsus unlike that of Birds was free, and was not an- 

 chylosed with the metatarsus. Huxley concludes that " it is 

 impossible to look at the conformation of this strange Reptile, 

 and to doubt that it hopped or walked in an erect or semi- 

 erect position, after the manner of a bird, to which its long 

 neck, slight head, and small anterior limbs must have given it 

 an extraordinary resemblance." 



CHAPTER XXXIV. 

 BIRDS. 



THE fourth class of the Vertebrata is that of Aves, or Birds. 

 The Birds may be shortly denned as being " oviparous 

 Vertebrates with warm blood, a double circulation, and a 

 covering of feathers " (Owen). More minutely, however, the 

 Birds are denned by the possession of the following char- 

 acters : 



The skull articulates with the vertebral column by a single oc- 

 cipital condyle. Each half or ramus of the lower jaw consists of 

 a number of pieces, which are separate from one another in the 

 embryo ; and the jaw is united with the skull, not directly, but by 

 the intervention of a quadrate bone (as in the Reptiles}. The fore- 

 limb in no existing birds possesses more than three fingers or 

 digits, and the metacarpal bones are anchylosed together. In all 

 living Birds the fore-limbs are useless as regards prehension, and 

 in most they are organs of flight. The hind-limbs in all Birds 

 have the ankle-joint placed in the middle of the tarsus, the proxi- 

 mal portion of the tarsus coalescing with the tibia, and the distal 

 portion of the tarsus being anchylosed with the metatarsus to con- 

 stitute a single bone known as the " tarso-metatarsus" 



The heart consists of four chambers, two auricles and two ven- 

 tricles ; and not only are the right and left sides of the heart com- 

 pletely separated from one another, but ther) is no communication 

 between the pulmonary and systemic circulations, as there is in 

 Reptiles. 



The respiratory organs are in the form of spongy cellular lungs, 

 which are not freely suspended in pleural sacs ; and the bronchi 



