AVES. 



BiKDS are distinguished from all other A^ertehrates by their 

 covering of featliers. Though related to the lieptiles, they differ 

 in heing warm-blooded — a feature which is correlated with a four- 

 chambered heart, iu which the chau)bers 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 iu the lieptiles, 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 rauii are never separable as in lieptiles and many Mammals. 

 Proximally the mandible articulates with the skull, after the 

 reptihau 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 liave become excessively 

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

 have become fused. The ilium lias 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 disappeai-ed altcygether, whilst iu the Ostrich 

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

 hand, and the primitive mammals Echidna and OrnWiorhynchns 

 on the otlier, 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, Hat'tfo' and Carinatce, according to the 



