AVES.
Birds are distinguished from all other vertebrates by their covering of feathers. 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 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 movement between the vertebræ within its grip. As a consequence, these vertebræ, 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 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, Ratitæ and Carinatæ, according to the
