198 BIRDS—DEFINITION OF CLASS. 
AVES—BIRDS. 
“ Birds are abranchiate vertebrates, with the brain filling the cranial 
cavity, the cerebral portion of which is moderately well developed, the 
corpora striata connected by a small anterior commissure (no corpus cal- 
losum developed), proscencephalic hemispheres large, the optic lobes 
lateral, the cerebellum transversely multifissured ; the lungs and heart 
not separated by a diaphragm from the abdominal viscera; aortic arch 
single (the right only being developed); blood with nucleated red corpus- 
cles, undergoing a complete circulation, being received and transmitted 
by the right half of the quadrilocular heart to the lungs for aeration 
(and thus warmed), and afterwards returned by the other half through 
the system (there being no communication between the arterial and 
venous portions); skull with a single median convex condyle, chiefly on 
the basi occipital (with the sutures for the most part early obliterated); 
the lower jaw with its rami ossifying from several points, connected 
with the skull by the intervention of a quadrate bone (homologous 
with the malleus); pelvis with ilia prolonged in front of the acetabu- 
lum, ischia and pubes nearly parallel with each other, and the ischia 
usually separated; anterior and posterior members much differentiated ; 
the former modified for flight, with the humerus nearly parallel with 
the axis of the body and concealed in the muscles, the radius and ulna 
distinct, with two persistent carpal bones, and two to four digits; the legs 
with the bones peculiarly combined, (1) the proximal tarsal bones coales- 
cing with the adjoining tibia, and (2) the distant tarsal coalescing with 
three (second, third, and fourth) metatarsals (the first metatarsal being 
free), and forming the so-called tarso-metatarsus; dermal appendages de- 
veloped as feathers; oviparous, the eggs being fertilized within the 
body, excluded with an oval calcareous shell, and hatched at a tempera- 
ture of about 104° F., (generally by the incubation upon them by the 
mother).”—(GILL.)* 
There is little to add to this almost exhaustive definition of the class. 
All of our birds, at least, have the jaws more or less covered with a horny 
membrane, and all have air spaces connected with the lungs, and the 
skeleton itself is more or less pneumatic. 
All existing birds may be placed in two great groups or sub classes— 
Ratite, birds without a keeled breast-bone, ostriches, etc., and Carinatz, 
those with a more or less well developed keel to the sternum. All North 
American birds belong to the latter group. | 
* North American Birds, Vol, I, xi. 
