VERTEBRATA. 3827 
on the same level. The eggs are usually colored. Here 
belong the Ravens, Crows, Jays, Birds of Paradise, Black- 
birds, Orioles, Larks, Sparrows, Tanagers, Wax-wings, Swal- 
lows, Wrens, Warblers, Thrushes, ete. 
Crass V.—Mammalia. 
Mammals are distinguished from all other creatures 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-concaye; and either a 
part or whole of the body is hairy." 
They are all warm-blooded Vertebrates, breathing only 
by lungs, which are suspended freely in the thoracic cavi- 
ty; 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 being circular (except in the Camel); the 
entrance to the windpipe is always guarded by an epiglot- 
tis; 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 cere- 
bellum has lateral lobes, a mammalian peculiarity ;° the 
cranial bones are united by sutures, and they are fewer 
than in cold-blooded Vertebrates; the skull has two oc- 
cipital condyles, a feature imitated only by the Amphib- 
ians; the lower jaw consists of two pieces only (often uni- 
ted), and articulates directly with the cranium; with two 
exceptions (Manatee and Hoffman’s Sloth), there are al- 
ways seven cervical vertebrae; the dorsals, and therefore 
the ribs, vary from ten to twenty-four; the articulating 
surfaces of the vertebrae are generally flat; the front 
limbs are never wanting, and the hind limbs only in a 
few aquatic forms; excepting the Whales, each digit car- 
