AVES 
Birps are distinguished from all other vertebrates by their 
covering of feathers s. Though related to the Reptiles, they differ 
in being warm-blooded—a feature which is correlated with a four- 
chambered 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 Reptiles, 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 Reptiles and many Mammals. 
Proximally tne 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. ‘lhe ilium has become greatly elongated, and 
is closely applied to the vertebral column, preventing “all move- 
ment between the vertebre 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 i eres bone” as in Dinosaurs, while the fourth has 
become reduced to a mere nodule of bone supperting the hallux. 
Jn 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 Eehidna and Ornithorhynchus 
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, Ratite and Carinate, according to the 
