558 ZOOLOGY. 
is in the nature of the limbs, the structure of the head, th 
organs of special sense, together with the increased con 
plexity of the teeth, and the size and complicated structur 
of the brain, particularly of the cerebrum and cerebellum. 
The more important (diagnostic) features of the mamma: 
are the articulation of the lower jaw directly to the skul. 
the quadrate bone becoming the zygomatic process of th 
squamosal; there are two occipital condyles; the teeth are di: 
ferentiated into incisors, canines, premolars, and molars; th 
‘body is covered with hair.* The body-cavity is divided int 
two compartments (thorax and abdomen) by a large muscle 
the diaphragm, so that the lungs are separated from the ab 
dominal viscera. From the four-chambered heart th 
single aorta is reflected over the left bronchus ; the blood i 
warm, with non-nucleated corpuscles ; the circulation is com 
plete, the blood being entirely received by the right auricl 
and transmitted by the right ventricle to the lungs for aéra 
tion, whence it is afterward returned by the left ventric] 
through the system. The brain is much larger than 
birds, the cerebral hemispheres forming the bulk of th 
brain, and gradually, in different members of the ascendin; 
series, overarching and finally concealing from above th 
cerebellum. The cerebral hemispheres are more or les 
connected (and in nearly inverse ratio) by an anterior com 
missure and a superior transverse commissure (corpus callo 
sum), the latter more or less roofing in the lateral ventricle 
(Gill). Mammals are viviparous, the embryo developin, 
from a minute egg, and the young after birth are fed b 
the mother with milk secreted in the mamme or mammar 
glands; hence the name of the class, Mammalia. 
Returning to the skeleton, which we may examine mor 
in detail : the skull, as a brain-box, is much larger than h 
the reptiles and birds. The brain-cavity of Coryphodo: 
and other extinct Tertiary mammals was exceedingly small 
scarcely larger i in proportion than in reptiles, and there is 
progressive increase in size of the cavity of the skull in th 
more specialized descendants of this early Tertiary type, a 
seen in that of the horse, when compared with its Eocen 
progenitors. There is also a decided increase in the brain 
* The incus and malleus bones are also diagnostic of mammals. 
