DEFINITION OF BIRDS. 
61 
are any living members of the class of Birds from any known Reptiles, the characters of the 
two groups converge in geologic history so closely, that the presence of feathers in the former 
class, and their absence from the latter, is one of the most positive diflferences we have found. 
The oldest known birds are from the Jurassic rocks of Europe, and the Cretaceous beds of 
North America. These birds had teeth, and various other strong peculiarities of structure, 
which no living members of the class have retained. 
AVES, or the Class of Birds, may be distinguished from other Sauropsida, for all that 
is known to the contrary, by the following sum of characters : The body is covered with 
feathers, a kind of skin -outgrowth no other animals possess. The blood is hot ; the circu- 
lation is completely double ; the heart is perfectly four-chambered ; there is but one (the right) 
aortic arch, and only one pulmonary artery springs from the heart ; the aortic and the pulmo- 
nary artery have each three semilunar valves. The lungs are fixed and moulded to the cavity 
of the chest, and some of the air-passages run through them to admit air to other parts of the 
body, as under the skin and in various bones. Reproduction is oviparous ; the eggs are very 
large, in consequence of the copious yolk and white ; have a hard chalky shell, and are hatched 
outside the body of the parent. There are always four limbs, of which the fore or pectoral 
pair are strongly distinguished from the hind or pelvic pair by being modified into wings, 
fitted for flying, if at all, by means of feathers — not of skin as in the cases of such mammals, 
reptiles, and fishes as can fly. The terminal part of the limb is compressed and reduced, 
bearing never more than three digits, only two of which ever have claws, and no claws 
being the rule. There are not more than two separate canrpals^ or wrist-bones, in adult recent 
birds (with very rare exceptions) : nor any distinct interclavicular bone. The clavicles are 
complete (with rare exceptions), and coalesce to form a ''wish-bone" or "merry-thought." 
The sternum, or breast-bone, is large, usually carinate, or keeled, and the ribs are attached to 
its sides only ; it is developed from two to five or more centres of ossification. The sacral ver- 
tebrae proper have no expanded ribs abutting against the ilia ; the ilia, or haunch-bones, are 
greatly prolonged forward ; the socket for the head of the femur fOv thigh-bone^ is a ring, not a 
cup ; the ischia and piibes are prolonged backward in parallel directions, and neither of these 
bones ever unites with its fellow in a ventral symphysis (except in Struthio and Rhea). The 
fibula, or outer bone of the leg, is incomplete below, taking no part in the ankle-joint. The 
astragalus^ or upper bone of the tarsus, unites with the tibia^or inner bone of the leg, leaving 
the ankle-joint between itself and other tarsal bones, the lower of which latter similarly unites 
with the bones of the instep, or metatarsus. There are never more than four metatarsal bones, 
and the same number of digits ; the first or inner metatarsal bone is usually free, and incom- 
plete above ; the other three anchylose (fuse) together, and with distal tarsal bones, as already 
said, to form a compound tarso-metatarsus. Recent birds, at any rate, have a certain saddle- 
shape of the ends of the bodies of some vertebrfe. Such birds have also no teeth and no fleshy 
lips ; the jaws are covered with horny or leathery integument, as the feet are also, when not 
feathered. 
The Position of the Class Aves among other Vertebrates is definite. Birds come in 
the scale of development next below the Class Mammalia, and no close links between Birds 
and Mainmals are known ; the most bird-like known mammal, the duck-billed platypus of 
Australia {Ornithorhynchus paradoxus), being several steps beyond any known bird. Birds 
are the higher one of the two classes of Sauropsida — the lower class, Reptilia, connecting with 
the Batrachians (frogs, toads, newts, etc.) and so with the Fishes, Ichthyopsida. In this Verte- 
brate series. Birds constitute M^hat is called a highly specialized group ; that is to say, a very par- 
ticular off-shoot, or, more literally, a side-issue, of the Vertebrate genealogical tree, which in 
the present geological era has become developed into very numerous (about 10,000) species, 
