AVES. 



BIRDS are distinguished from all other vertebrates by their 

 covering of feathers. Though related to the Keptiles, they differ 

 in being warm-blooded a feature which is correlated with a four- 

 chainbered heart, in which the chambers are completely separated, 

 thus preventing the intermixture of arterial and venous blood 

 which obtains among the lower vertebrates. Of the right and 

 left aortic arches present in the Eeptiles, only the right persists 

 in Birds and the left in Mammals. The skull, which presents no 

 sutures in the adult, possesses but a single occipital condyle and 

 the jaws are produced into a beak ensheathed in horn, whilst in 

 more primitive, extinct species, they were armed with teeth. 

 The lower jaw is a complex of several bones, but the right and 

 left rami are never separable as in Keptiles and many Mammals. 

 Proximally the mandible articulates with the skull, after the 

 reptilian fashion, by means of a quadrate bone. The fore-limb 

 has become transformed into a " wing," and the sternum, in 

 accordance with the requirements of flight, has taken on the form 

 of a broad, oblong plate, usually provided with a median keel for the 

 attachment of the pectoral muscles, which have become excessively 

 developed. In the hip-girdle the three elements of the pelvis 

 have become fused. The ilium has become greatly elongated, and 

 is closely applied to the vertebral column, preventing all move- 

 ment between the vertebrae within its grip. As a consequence, 

 these vertebrae, which include more or fewer of the lumbar, the 

 sacral and a variable number of post-sacrals, have become welded 

 together to form a synsacrum. In the hind-limb the proximal 

 row of tarsa's have become fused with the shaft of the tibia to 

 form a " tibio-tarsus," while the distal row have fused with the 

 metatarsals to form a tarso-metatarsus. On this account the 

 ankle-joint is " intertarsal " as in many reptiles. Three of the 

 four surviving metatarsals have fused to form a solid, cylindrical 

 shaft or "cannon-bone" as in Dinosaurs, while the fourth has 

 become reduced to a mere nodule of bone supporting the hallux. 

 In many species the hallux has become reduced to a mere vestige, 

 and, in some, it has disappeared altogether, whilst in the Ostrich 

 (Struthio) but two toes remain. With the reptiles on the one 

 hand, and the primitive mammals Echidna and Ornitlioi-hynchus 

 on the other, birds agree in being oviparous. 



Hitherto most systems of classification have been founded on 

 living birds only, and have therefore to some extent failed in their 

 purpose. Birds have been commonly divided into two great 

 groups or sub-classes, Ratitce and Ccirinatce, according to the 



