340 
COMPARATIVE ZOOLOGY. 
agers, Wax-wings, Swallows, Wrens, Warblers, Thrushes, 
etc* 
Class Ye —Mammalia. 
Mammals are distinguished from all other Vertebrates 
by any one of the following characters: they suckle their 
young; the thorax and abdomen are separated by a per¬ 
fect diaphragm; the red corpuscles of the blood have no 
nucleus, and are therefore double-concave; and either a 
part or the whole of the body is hairy at some time in 
the life of the animal. 172 
They are all warm-blooded Vertebrates, breathing only 
by lungs, which are suspended freely in the thoracic cav¬ 
ity ; the heart is four-chambered, and the circulation is 
double, as in Birds; the aorta is single, and bends over 
the left bronchial tube ; the large veins are furnished with 
valves ; the red corpuscles differ from those of all other 
Vertebrates in having no nucleus and in being circular 
(except in the Camel); the entrance to the windpipe is 
always guarded by an epiglottis; the cerebrum is more 
highly developed than in any other class, containing a 
greater amount of gray matter and (in the higher orders) 
more convolutions; the cerebellum has lateral lobes, a 
mammalian peculiarity, and there is a corpus callosum 
and a pons varolii / the cranial bones are united by 
sutures, and they are fewer than in cold-blooded Verte¬ 
brates ; the skull has two occipital condyles, a feature 
shared by the Amphibians; the lower jaw consists of 
two pieces only (often united), and articulates directly 
with the cranium; w T ith four exceptions there are always 
seven cervical vertebrae ; 173 the dorsal vertebrae, and there¬ 
fore the ribs, vary from ten to twenty-four ; the lumbar 
vertebrae number from two to nine; the sacral from three 
to nine, and the caudal from two to forty-six ; the articu¬ 
lating surfaces of the vertebrae are generally flat; the 
fore-limbs are never wanting, and the hind-limbs only in 
