57i 



DIVISION ILSAUROPSIDA. 



CHAPTER LXIII. 



CLASS IV. A VES. 



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

 The Birds may be shortly defined as being " oviparous Verte- 

 brates with warm blood, a double circulation, and a covering 

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

 defined by the possession of the following characters : 



The embryo possesses an amnion and allantois, and branchiae 

 or gills are never developed at any time of life upon the visceral 

 arches. The skull articulates with the vertebral column by a 

 single occipital condyle. The form of the vertebral centra 

 varies ; but they are in no case amphiccelous, except in the 

 remarkable extinct form described under the name of Ichthyor- 

 nis. 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-j 

 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 proximal portion of the tarsus coalescing with the tibia, 

 and the distal portion of the tarsus being anchylosed with the 

 metatarsus to constitute a single bone known as the " tarso- 

 metatarsus." 



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

 ventricles ; and not only are the right and left sides of the 

 heart completely separated from one another, but there is no 

 communication between the pulmonary and systemic circula- 

 tions, as there is in Reptiles. There is only one aortic arch, 

 the right. The blood is hot, having an average temperature of 



