558 ZOOLOGY. 



is in the nature of the limbs, the structure of the head, the 

 organs of special sense, together with the increased com- 

 plexit}' of the teeth, and the size and complicated structure 

 of the brain, particularly of the cerebrum and cerebellum. 



The more important (diagnostic) features of the mammals 

 are the articulation of the lower jaw directly to the skull, 

 the quadrate bone becoming one of the ear-bones (the mal- 

 leus) ; there are two occipital condyles ; the teeth are differ- 

 entiated into incisors, canines, premolars, and molars ; the 

 body is covered with hair. The body-cavity is divided into 

 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 the 

 single aorta is reflected over the left bronchus ; the blood is 

 warm, with non-nucleated corpuscles ; the circulation is com- 

 plete, the blood being entirely received by the right auricle 

 and transmitted by the right ventricle to the lungs for aera- 

 tion, whence it is after ,vard returned by the left ventricle 

 through the system. The brain is much larger than in 

 birds, the cerebral hemispheres forming the bulk of the 

 brain, and gradually, in different members of the ascending 

 series, overarching and finally concealing from above the 

 cerebellum. The cerebral hemispheres are more or less 

 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 ventricles 

 (Gill). Mammals are viviparous, the embryo developing 

 from a minute Q.gg, and the young after birth are fed by 

 the mother with milk secreted in the mammre or mammary 

 glands ; hence the name of the class. Mammalia. 



Returning to the skeleton, which we may examine more 

 in detail : the skull, as a brain-box, is much larger than in 

 the reptiles and birds. The brain-cavity of Curypliodon 

 and other extinct Tertiary mammals was exceedingly small, 

 scarcely larger in proportion than in reptiles, and there is a 

 progressive increase in size of the cavity of the skull in the 

 more specialized descendants of this early Tertiary type, as 

 seen in that of the horse, when compared with its Eocene 

 progenitors. There is also a decided increase in the brain- 



