340 COMPARATIVE ZOOLOGY. 



agers, Wax-wings, Swallows, Wrens, Warblers, Thrushes, 

 etc. 



CLASS Y. 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 ; with 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 



